Sills are crucial. Have a really good look. They are in 3 parts so the inner will be well gone if there's any rot visible on the outer. Feel inside front wheel arches. There is an inner piece above the wheel which forms a sort of shelf and can collect moisture. If it's solid there's a good chance the rest of the car is ok. There are some splash panels behind the front wheels inside the wings. They are removable and give you a view of the front of the sills, bulkhead and A posts. Depends if he owner will let you take them off... Check rear wheel arches for holes - if you can get the carpet up that can really help. Check door hinges and A posts for rot. If it's a GT (sounds like it is) check the rear corner of the rear side windows. Water collects here, rots through and then dips down onto wheel arch inners. If you can get at the floor, have a good look. They are often patched up as any leaks soak into the carpets. Have a good look at the bulkhead with the bonnet up. Check that the doors and rear tailgate seal properly and have reasonably consistent gaps. My tailgate doesn't seal properly because it's misshapen. Bodywork is the crucial thing but mechanically it's good to know that the diff isn't noisy and the overdrive works (switch can be to blame if it isn't though it could be more serious). Engines are pretty bullet proof (and sound bloody lovely). I'm sure there might be other things I haven't thought of.