The first thing to look for is a good Porsche mechanic to do the PPI (Pre-Purchase Inspection). You may have owned a flat-6 before but mechanics with years of experience will know so much more. Best couple of hours of mechanic's time I ever paid for. Knew what it needed and had a plan to address the issues. And that car (my third Porsche) was a third as old as what you are looking at and probably lower mileage.
https://sites.google.com/site/mikefocke2/mikesporscheboxsterwebpages has things like a list of model year changes and, while written years ago, may help.