Andrew,
In my humble opinion, if you are going to make any compromises it should not be on the structure of the car and the interior. Both can be far more expensive to restore than any issues with the engine and transmission.
I suspect the 383 and 400 engines in the 411s have better parts availablity than the 318 engine in the 410, but not enough difference to warrant avoiding the 410.
Another thing to pay attention to is the front suspension. There's a bit of information on the potential problem here
http://www.bristolcars.info/forums/8...ont+suspension
The distance between the bottom of the front crossmember and the road should be about 5 inches on a 411 (this does vary a little depending on the tyres fitted).
If a car has been standing for a long time or has no record of the brakes being overhauled, then that is also a likely requirement, although that could be said for almost any 50+ yo car.
Also, lift the vinyl cover on heater box and check for rust in the tray beneath the Smiths heater box.