Basics

 The Sunbeam bodyshell, like most of the rest of the car, is based on the same floorpan as the Hillman Avenger. It suffers from the same kind of foibles as most mass production cars of that era. Bodyshells vary quite a bit in size, and the dreaded tin worm certainly has a taste for the car. Most of this information relates to the Sunbeam, but I doubt if there's much difference between this and the Avenger, especially the post face lift (series 7 ownards) models.

Critical Measurements

 Err, if someone would like to provide me with these (they're probably in the Chrysler workshop manual, which I don't have a copy of), I'll put them in.

Weak Points

 The Talbot Sunbeam bodyshell, like most cars produced in that era, is fairly suceptible to rust. And where it rusts, it falls apart if you use the car hard. The common places that most Sunbeam bodyshells rust and generally fall apart are:
Outer Sills
Once moisture gets into these, or the paint gets damaged, they tend to rot out. In common with the inner sills, I've seen this more often at the back than the front, especially at the bottom where any moisture tends to sit. Also watch out for it where the sill panel joins the rear quarter panel. When I had to do these on my second Sunbeam, they were reasonably readily available as repair panels, but I've heard tell (late '99) that they're getting difficult to get hold of.
Inner Sills
These rust quite badly, normally from the back to the front, and the bit inside the front jacking point. If you're doing the outer sills, then expect to have to do some repair work on these as well. They're not available as replacements, but since it's only flat sheet steel, they're not difficult to fabricate.
Front Jacking Points
I've never actually seen one of these which is in a condition where it will support the weight of the car. Inside the jacking point, directly above the hole which locates the standard jack, is a brace which seems to be the first thing which goes, closely followed by the face which supports the weight of the car on the jack. They might be available again, but they're not difficult to fabricate from sheet steel.
Struct Tops
Another popular place for failing the MOT, probably because it's a nice easy one to spot. Moisture and muck get caught between the strut tower and the inner wing panel at the top, and then it simply rusts through the inner wing. Normally, repair panels for these come as part of a plating kit, but I think that getting hold of repair panels by themselves is starting to get tricky. They're probably not too tricky to make out of sheet steel, and they're certainly not difficult to fit.
Seam between the inner wing and the bulkhead, at the top
The acute angle between the bulkhead and the rear edge of the inner wing tends to trap moisture and muck, and it then rusts through at the top. Quite an easy place to repair, and a popular one to strengthen on top as well. Some plating kits include a section which triangulates this joint from underneath, thus stopping a lot of the muck getting trapped there.
Headlight boxes
These aren't structural items, but don't be surprised if you take the wings off and find that the have simply vanished, leaving a small, rust bitten plate attached to the wing with some very rusted in bolts. You can repair them with pretty much anything - something which doesn't rust is probably advisable - but it is worth repairing them since they stop the muck from under the wheel arches attacking the headlight wiring.
Front Panel
The lower edge of the front panel, and the edges where it joins to the wings, tends to rust away for some reason. I've never quite worked out why, but it's probably because it gets covered in muck from behind. The outer edges are quite easy to repair, either with a bit of sheet steel or even just aluminium riveted in place. I've also seen fibreglass front panels, but (personally) I'm not convinced by these since they do take quite a few knocks when rallying and I'd guess that they break quite easily. Also keep an eye on where the chassis rail mounts to the front panel. The front panel takes a lot of the braking effort - it's what stops the chassis rails pulling apart - and I've failed the MOT on this before.
Rear wheel arches
The seam where the inner arch joins the outer arch seems to attract and hold muck quite nicely, and then both panels rust out. Normal practice is to fold the seams up inside the arch to prevent wider tyres rubbing, but this can simply make the gap bigger and exacerbate the problem. I was lucky enough that my current shell is actually in very good condition here, and I've fibreglassed over the join to try to stop it filling and holding muck. My previous shell rusted quite badly here, and I ended up having to put a lot of fibreglass and filler in when I tried to tidy this area up. I have seen repair panels being sold for Sunbeams, but rumour has it that they are actually Ford Escort repair panels which can be bodged into place.
Lower rear quarter
The lower rear quarter, at the bottom behind the rear wheels, would appear to be designed to collect muck and convert it to rust. These are not too tricky to fabricate, although to get a good result you might want to find someone who's practiced at getting the contours correct. Alternatively, you could just put a flat bit of metal in place and make the rest from filler...
Front chassis rail
The front chassis rail is particularly likely to rot out between the drag link mounts and the cross member mount, probably because of the stresses involved in braking. Fortunately this is one of the bits which normally gets reinforced when preparing a shell, and having done this I've never had any problems.
Also keep an eye on where the drag link mounts are welded to the chassis rails as these can also rust out.
Front footwells
Especially prone to rust if you have carpets in to trap moisture, the front footwells seem to collect any water which comes in round the screen or through the windscreen wiper mounts (or any additional rust holes you may have!). When I broke my first Sunbeam, we found sedimentary rock formations under the carpet (and the colours of the muck showed the different events that the car had been on!). They're fairly easy to repair with sheet steel.
Rear footwells
Any moisture which doesn't get caught in the front footwell probably drains down to the rear footwell. I've only seen my second shell go here, and it was already gone when I bought the shell ("surface rust mate, only need to paint over it"). It's worth keeping an eye on it since its what the roll cage feet push against... Pretty easy to plate over with sheet steel.
Rear mounts on the fuel tank
Most Sunbeam spare wheel wells collect moisture, and the lowest point of this areas on each side is where the fuel tank mounts. When we broke my first shell, we just pulled hard and the tank came out, complete with mounts. Worth keeping an eye on, and rust free.
Rear panel
I've seen some shells go along the rear panel, just beneath the rear window seal. Never known why, and probably a pig to repair as well.
Seat mounts
The standard seats, with their half inch or so diameter spacers, tend to start to separate the standard reinforcing plates from beneath the floor, and then work their way through. Easily repaired with additional plating on the inside.
Rear vents
Behind those removable rear vents is a beautifully designed rust trap. I doubt that it is at all structural, but you'll probably find that once it has rusted through the water simply drains down to rot out either your sill or your rear quarters. Definitely worth a bit of filler to eliminate this possibility.
Rear Damper upper mounts
Especially when you're using stiffer rear dampers, the rear damper mountings are prone to trying to pull themselves away from the floor. There is a reinforcing plate under the floor on each side, but this too tends to pull away. The tell-tale signs are the spot welds around them pulling away. It's worth seaming the damper mount to the floor.

Other places I've seen go, which may be peculiar to my cars, are:

Rear chassis rails
This is probably due to too many hard jumps/landings. I found that one of the chassis rails had cracked most of the way round, behind where it goes over the axle. I'd have guessed that this is where it takes the weight of the boot floor.
Bulkhead under the front wings
I was quite surprised to find a two inch long crack in the bulkhead, outside the inner wings. I suspect that the tin worm had been working its evil ways inside some paint damage, and over the years it had gone unchecked.
C post to roof
I'm not quite sure why this started rusting, but when I investigated it I found that I'd got a hole nearly two inches in diameter where the C post joins the roof. I suspect that moisture comes in here and drains down to collect in the lower rear quarter...

Stitch Welding

 Like preparation on most cars, the Sunbeam benefits (in length of service, if nothing else) from stitch welding most seams you can find. I've no factual data on which bits are the most worthwhile, but the following is a list of the areas I've done in the past:

Strengthening and Plating

 There are a number of plating kits available for Sunbeams, which plate over a lot of the common problem areas as well as areas which need reinforcing for motorsport purposes.

Plating kits normally cover the following:

Strut top mounts
Essentially just a repair panel to weld on, regardless of whether the shell needs it or not. Has the side effect of covering up the chassis number stamped onto the offside strut top...
Double skinning the front chassis rails
I reckon that this has to be one of the most worthwhile exercises given the amount of stress these go through in a motorsport car. Some kits have a number of separate pieces, which probably make them easier to fit, whilst others (PTS ones, I think) come as one long piece which goes right from the front of the car to the jacking point.
Top of the inner wing to the bulkhead
I've seen these in two different forms. The first, which is more simple to fit, simply sits on top of the joint (next to the bonnet hinge). The second, which I've never personally fitted, triangulates the joint from underneath. This has the side effect of preventing too much muck and moisture getting into the joint from underneath and rotting it out. I'd say that this was the better type, although there's nothing to stop you doing both.
Front chassis rails to bulkhead
Most kits come with a right angled 'u' section which fits between the top of the chassis rail and the bulkhead at the back of the engine bay. I once looked at fitting one of these, and decided that it was going to be nigh on impossible to get the welder in there (even without the engine in place) to attach it soundly. So I made my own triangular plate and boxed it in like that.
Front of rear trailing link mounts
These short box sections, which have the rear lower suspension arms attached to them, are quite vulnerable, especially on a forest car. Most plating kits have some form of protection for these.
Over the years, I've also found that plating or strengthening the following areas also makes a lot of sense. Some of the plating for these areas comes from plating kits, and some is from experience.
Front chassis rails to front bulkhead
The triangular plates, which come with some plating kits, lie flat inside the engine bay at the front of the chassis rails where they meet the section which runs under the radiator.
Rear floor to inner wheel arches
The joint between the rear floor and the rear inner arches seems to be quite a weak one, which I've seen problems with on a number of cars. You could see daylight through this joint on one car I saw! It's worth putting some triangulation between the floor and the arches to stop this moving quite so much, and I've fitted what is essentially a rear strut brace across the floor to help matters as well. A common sign that you have problems here is that there is muck or water getting into the spare wheel well, but you can't find where it's coming from.
Seat mounts
The standard seat mounts are quite weak, and you normally find that the standard seats have started to punch through the floor. It's worth putting additional plating on the floor before refitting even the standard seats. If you're putting after market seats in, and especially if you're long in the legs, you'll find that you need to put additional supports in above the rear footwell to support the seat. I did this by welding some box section in across the footwell and mounting the seat frame to this.
Front Strut Brace
A common fitment, readily available from many suppliers and also simple to fabricate.
Sills
I've seen one Sunbeam where they had boxed the inner sills in, so that they joined the edge of the floor rather than the top of the sill. I reckon that this probably made it a bit stronger, stopped too much muck getting in there and also made it easier to weld in. Mind you, the owner worked at a car bodyshop and thus had access to all the right kit for doing this sort of thing.

Roll Cages

 There are many different types, makes and shapes of roll cages out there. Safety Devices are known for making some very good cages, but they are also a bit more expensive than the rest. I've got a Roll Centre cage in my car, and although it was a bit of a fight to get it assembled and the A post bar is in a very inconvenient place in relation to the standard door handle, I've got no complaints so far. My previous car had a second hand (steel) Alley Bars cage in, and that was a real pig to fit. But the previous owner of the cage said that it had gone straight into his car, so that just shows how much the shells differ. It was still straight when it went into its next owners car, even after having been "used"! I've not seen any weld in cages for Sunbeams like you can get for some cars, but I'm sure that if you paid someone enough they'd make one for you.

I have multipointed the cages on both my cars, and believe that this adds quite a bit of strength to the shell. Most roll cage manufacturers sell the kit to do this with the cage. When I first fitted the cage to my second shell, it was just mounted on the standard six points, and it was amazing how much movement you could feel between the roof and the cage at the top of the door aperture, even when you were just driving down a normal road. My current car is multipointed at the top and half way down both the 'A' post and the 'B' post. The previous cage was multipointed (if I remember correctly) at the top of the 'A' post and from the 'B' post of the cage to the seatbelt mount in the cars 'B' post.

I've seen a couple of cars where the cage has been tied in to the strut top mounts, by one or two bars going forwards through the bulkhead from the 'A' post, but in both cases this was an add-on that the owner had fabricated rather than it being a kit of any sort. This is something I'd reckon is pretty worthwhile, even if it is only to prolong the life of the shell.

Bonnet, Doors, Windows etc.

Windows

Perspex windows are a common and easy option for Sunbeams. There are a number companies who advertise in Motoring News who will sell you precut windows, alternatively you can make them quite easily with a jigsaw from a sheet and a half of 6 foot by 4 foot perspex. Making the sliders is a little tricky since the door frame curves which makes the sliders quite stiff.

It has been observed that for the rear quarter windows, the rubber sealing strip weighs nearly as much as the glass window. One option is to cut the perspex a little larger, and rivet it to the mounting lip (use a thin strip of aluminium to avoid cracking the perspex.

Mounting the windows in the doors is a little more tricky. I ended up using a strip of aluminum folded round the bottom and bolted through. The strip is then long enough on the inside to fold over onto the inside of the door and secure with a self tapping screw. However you mount these, it's definitely worth ensuring that the door waist seal is assisted with some clear silicone sealant - stops the water getting in and rotting out the bottom of the door.

There's quite a lot of weight to be saved in the tailgate. It's far and away the easiest thing to replace. Many people use bonnet clips or pins to secure it shut, using padlocks when it needs to be locked. I took a simpler option, and just used the standard catches, which I reckon are easier to fit and quicker to use in a hurry. I put a couple of plastic draw handles (a quid or two from most DIY stores) on the outside to help open it. Obviously the hydraulic props have to be done away with since they're too powerful for the perspex, but a bonnet prop can easily be fitted to provide the same function.

Doors

With the glass windows replaced with perspex, most of the innards of the doors can be cut out with a jigsaw or a nibbler. It's worth using a sheet of aluminium to box the inside of the door in, because otherwise it will end up being too flimsy to keep on the catch. It's probably also worth putting a strengthening brace inside this as well - the additional weight is minimal.

The only drawback to having a nice aluminium sheet is when the spring breaks on the door catch and you've got to take it all off again to replace the door striker mechanism and spring...

Bonnet

There are a number of schools of thought on the bonnet. One says to use a normal bonnet, since its strength is useful when you hit things. Another says to use a bonnet which is almost completely stripped out, saving as much weight as possible. If this is your chosen route then almost all the strengthening frame can be removed with an angle grinder, leaving just the outer skin and the bars down the outside to give it a bit of form. My second car came with a fibreglass bonnet, which did me fine. It also survived the weight of the car on its front edge when I made a small error of judgement in how fast I could get round a corner...

Bumpers

Most people use standard bumpers, although some people prefer to run without them altogether. Fibreglass bumpers are also available. The main discussions on which to run centre around the amount of protection for the rest of the front of the car is provided by the standard bumper. One of the arguments for running without a bumper is to aid the flow of air to the radiator, and I've also seen people cut holes in the lower portion of the bumper for the same purpose.

It's also useful to have a bumper if you are competing in an event which requires auxiliary lighting.

Another reason for not using the standard setup is the difficulty in getting hold of bumper end caps. They are normally the first thing to vaporise when having any sort of moment, and are also prone to simply falling off if not secure. I reckon that there is a market here for fibreglass ones...

Wings

The standard steel wings are particularly prone to rotting away at all corners. I think that new steel wings are available from a small number of suppliers (check the Avenger/Sunbeam owners club web site for details). They're obviously a lot stronger than the alternative fibreglass wings, but weigh more. Availability may be a stronger reason for choosing one over the other, as opposed to personal preference. Having been in a car which was physically put back on the track by spectators, I can definitely see the advantage in having steel wings, but that's probably for you too choose, based on your motorsport discipline.