Hi Phil
Having just bought one, my first Porsche. Here are my thoughts:
Try and find one with sports (wing back) seats, they are more comfortable, and grip in all the right places.
PSE is great. I had no idea how much it would add to my enjoyment of the car!
Check the Radio etc. My Bose unit gave up fairly quickly (the amp), and looking that the amount for sale on e-bay etc, I think this is a common problem.
Make sure the air con works, and that the radiators/condensers have been cleaned out - these cost a lot to sort.
Go for a lighter interior colour - mine is teracotta red, with chrono dials and silver console. It always make me smile when I get in. A black/black interior does not have the same sense of occasion.
Good luck, and happy hunting
Paul