As above, always ensure that you are covered by a PORSCHE warranty, not a third party product.
If the car is less than 2 years old it will have the original manufacturers warranty, thereafter it can be renewed annually. If you are buying a car that is out of the original warranty and doesn't have the Porsche extended one then you are going to need to get a 111 point check done by Porsche before you buy the car which will a) make sure that all is in order for some peace of mind and b) make the car elligible for a warranty. If any private seller or dealer refuses to let the car go in for a 111 point check then walk away as they might have something to hide.
or.... buy from a dealer like Stirlings who do all this for you.
I've been driving my 997 for 15 months now and aside from a small issue with the tracking system (human error by the operating company) and a puncture (totally my fault) it has never caused me a single problem. Rock solid. Note, on both of the issues above, a Porsche warranty includes access to Porsche Assistance (RAC) for free so you can call someone out even just for a puncture.
Also, if you buy on finance, you have some consumer protection built in which makes the finance company partly responsible for merchantable quality of the goods.