Mike Focke posted two articles on his site related to buying a Boxster. Here's the URL:
https://sites.google.com/site/mikefocke2/mikesporscheboxsterwebpages.
I'd also suggest a dealer PPI using the Porsche diagnostic tool to check for trouble codes, over-revs, camshaft variances at idle, etc. An independent shop with an Autologic or a Durametric diagnostic tool can do many of these same tests.
Good luck.