Definately drive a couple for comparison. 
  
Make sure it has the correct servicing- . P1, P2, P1, P3 etc. 
  
Check clutch on the test drive as well as that will set you back about £1k to replace it. 
  
Ask owner what fuel it has been run on and if they have any receipts to prove it (worth a punt!). Must be on high octane only and NOT 95.