A rule of thumb that has always worked well for me when buying used is to make sure you have about 10-15% of the price paid available as "back up" or "mad" money for the new toy.
I've never had an aftermarket warranty or CPO car, and I guess you could say it would be a nice cushion should you need to fix something, or around here, want to make changes just for fun!
Good luck on your search. There are plenty of Boxsters available, it will just take some time
If you have some patience, some mechanical knowledge and some tools, maintenance items are certainly a Do It Yourself proposition to save both time and money.