Check Mike Focke's guides - parts 1 and 2. Also see his page problem area page. You can find these at
https://sites.google.com/site/mikefocke2/mikesporscheboxsterwebpages.
The best thing you can do after a close eye inspection and test drive is to get a PPI (pre-purchase inspection) done by a dealer or independent shop. If a dealer is selling the car, get the PPI done somewhere else.
It takes 20 to 30 minutes to take off and put back on the covers for the engine. There are two: under the convertible top and behind the seats. There isn't a way to directly check the IMS or RMS. To see them, you have to remove the transmission and flywheel. Occasionally, you can see the result of a leak in the IMS or RMS by noticing small drips, but it's almost impossible to tell which is the cause. Paperwork evidence of an IMS replacement probably means that it was done as a preventative measure. When the IMS fails, the engine destroys itself in the process. To replace the IMS and RMS, your probably looking at $1500 - $2000. If you decide to do that, then replace the clutch as well for another $500.
The best indicator of a quality car besides having a PPI and complete service records is finding one that is very clean all over