1. Is to know if you should buy it.
2. Is to know what you'd need to do to make it safe (if it isn't)
3. Is to have an idea of what you'll need to do in the future. How many miles to go on the tires, etc.
Unless the car is very low miles and you know the owner and have all the records, please do a PPI.
An extended warranty is only as good as the company behind the warranty. I bought an 01 2 years ago and had the chance to pick up the existing warranty or take a $3k lower price. I took the lower price. I've probably spent $500 that would have been covered by the warranty in 2 years. I'm ahead of the game. Understand that only about 20% of the $ you spend of an extended warranty for any item is planned by the insurer to go towards future repair costs, the rest is selling, administrative expense and profit. Now why go into a deal like that?
Of the warranty companies, Warranty Direct has a good reputation.
Might want to read some of these pages on buying a Boxster
http://mike.focke.googlepages.com/mikesporscheboxsterwebpages