Surely you can tell the mileage of the car from:-
Wear on the seats, carpet, pedals, Gearknob etc.
Stone chips (tiny) on windscreen.
Stonechips on bumper if not resprayed.
Previous MOT's?
Service history?
I wouldn't put too much weight on the condition of the exhaust as the car may have only done a low amount of miles but the mild steel (or tin!) of the exhaust will be rusting even as the cars sitting around.
Also the back box issue is a common problem with 350's.