I just bought one and still a newbie but I can provide some comments/guidance:
1. Leaks. Check for leaks between the transmission/engine. Boxsters are well known for having them on the Rear Main Seal (RMS) what in itself is a very small/affordable part but the amount of work to get it done (in case someone will do it for you) is not small so labor is expensive.
2. Clutch. If it needs a new one, perfect time to change the RMS so you do not pay for the exact same labor twice. Not that I need a new one right now but as it is relatively inexpensive (Clutch Kit from Sachs OEM, you can get around $350-400 - labor at least where I am is around $500).
3. Try to get someone to read whatever codes the car has/had on the computer log so you can get an idea of what it has been through.
4. The Mass Air Flow sensor (MAF) is another part that dies over time with heat on the engine. I know tons of people here keep cleaning it and putting it back but again, ideally it should be replaced probably every 40,000-50,000 miles. For a $300 part that can be replaced very easily, I see no point IMHO of cleaning it. I would not mind at all replacing it every 3-4 years (what means less than $10 a month). Check when this was replaced the last time.
5. Check if the car was modified to use a third party air filter for the intake. Some crazy people use oiled filters and they DO screw up the MAF. I put a new MAF last week and the old one was indeed all dirty/screwed up thanks to the oiled air filter. Now putting all the original air box back.
6. IMHO, if the owner has been treating this car as a Honda Civic, modding it and trying to get that extra HP out of it, I would walk away. As the one I got had only the air box changed (but the owner has every single original part he changed and complete service records) I thought it was ok for me. If the car had anything else changed that brings it to Honda Civic league mods I would NOT have bought it.
Again these are my own views/comments on the subject. Others may disagree completely with me. So take them with a grain of salt.
CR