It is not a deal killer but missing records is an issue: Make sure the mileage is correct! Run a carfax and make sure it supports the mileage. Also, make sure the car is not from a rust belt state. Even a newer, all galvanized car, will show signs of that. I
Sounds like you are all set for the PPI, so you will find out about the source of the oil leak. It generally is not adviseable to have a PPI at the shop that previously serviced the car, but in this case it may be okay, with the shop's reputation. The oil hardly ever comes from the case seam. That's just the lowest point where it usually ends up collecting.
It is a matter of taste, but that color is "rare" for a reason. Really a turn off for me, and I would think for many other buyers as well. If you like it, fine, but "rare" does not make it more desireable in this case and it will likely hurt resale value.
I was underwhelmed by the 5 speed and general power of a 92 C2 that I looked at. It was just barely even with my (hopped up) SC with a lot less throttle response at low speeds. You definitely will want to test drive the car in all road conditions. It had too much of a GT feel to it for me, with the stock suspension etc.
That said, think about what other cars are out there in that price range and do not fall in love with this particular vehicle. If you are set on a 964, drive others as well, so you can compare.
The price is too high. That car is a 15k car here in CA, with the mileage and the issues it has. Have I mentioned the color? I am sure that if you are serious and sit the owner down, you'll get it for 15. Negotiate hard! Be firm on your final offer and walk. Many sellers I have dealt with would call a week later and accept. I take my wife with me when I buy, she is ruthless. I hope I never get divorced, I'll be homeless ...
I hope my input is useful. Good Luck!
Cheers, George