Dave,
There are a lot of myths surrounding Bristols, one of them being that is very expensive to restore the suspension, but I have never heard that substantial bodywork is a common requirement.
I would say that Bristols are better than average compared with cars of similar age. The chassis on these cars is built to last, and significant rot in the chassis is not common. It can happen of course, if the car has been very poorly maintained, or stored in a field!
There are two specific places that the V8 commonly do suffer from rust. The front suspension is one place, specifically as shown in this thread
http://www.bristolcars.info/forums/showthread.php?t=40
The other place is where the rear shock absorbers mount onto the body. On my own car (411) this had been done before I bought the car, but I gather it is hard to repair without damaging the paintwork because of the welding that is involved, but perhaps others who have first hand experience could provide more detail on this.
As for the outer body, I have seem some corrosion at the top of the wheel arches but widespread corrosion of the body is not a common problem.
V8 Bristols are definitely no more expensive to restore than any other comparable classic car, and most of the crucial parts are available from Bristol Cars Services.
The bumpers bars were extremely well made, but they are no longer available so make sure they are in reasonable condition, or at least in a state in which they can be re-chromed. Windscreens can be expensive.
Bristol are now re-manufacturing original 411s, (using an original car as the basis), so maybe bumpers will be made again, but they will be very expensive. Best to buy one with good ones in the first place!
Contrary to the myth, the suspension on these cars is not incredibly complex, it is restorable at home, and most if not all of the parts are still available from Bristol Cars Services.
Incidentally, judging by what I have seen on the internet, the asking price of good Bristols seems to have risen markedly in recent years, like the rest of the classic car market. So you might see a 'correction' in the next 6-12 months.
These are of course just my opinions and hopefully a few other people will share their views.
Cheers,
Kevin