Check out Mike Focke's site:
http://mike.focke.googlepages.com/buyingorsellinga1997-2004boxster
My questions:
1. Do you have maintenance records? (let's see them, are they up to date)
2. Has it been in an accident? Repainted/touched up? (check paint, etc.)
3. Any leaks? (pay particular attention to the rear main seal)
4. Who maintained the car?
5. Are you the original owner? (if no, how long have they owned the car)
6. What issues, current or past, have they had with the car?
7. Will they allow the car to be independently inspected, at your expense (if no, run)
8. How many sets of keys do they have? (keys are expensive)
9. Cold weather climates only - did you store it the winter or drive it year round? F/U, if stored, where was it parked/stored - garage, outside, elsewhere? (inspect the location if possible - look for signs of leaks, etc.)
10. What do they like/dislike most about the car (this provides insight into it's care)
Always get a PPI done - it's the best insurance you can buy. Don't rely too much on Carfax - there are simply too many gaps.