I'd highly recommend getting a PPI (pre-purchase inspection) done by a qualified Porsche indy shop. They will know what to look out for, as far as red flags, and it should give you an idea of what condition it's in before you purchase it. There are also a lot of great detail about common problem areas/fixes for the 996 in our tech library here:
Pelican Parts Technical Articles - Porsche 996 & Porsche 997. I'd recommend reading up on a few of those.