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.