The 2 big sources of engine failure I have heard of:
1) Sleeve Slip Failure - Only on early models which have some engine defects "repaired" (instead of reworked to print). I think 1997 (USA) is the only year you need to avoid for this
2) IMS (intermediate shaft) failure - ALL models up to and including todays
Sadly, this crap is a worry we all must deal with and just hope it never happens. Following subsequent revisions to the design of the engines (minor tweaks/revs) it is clear porsche knows of the defect and its possible affects, yet they make no real acknowledgement and provide on official relief. (Think about what mazda did for almost all the RX8s).
Makes me strongly consider never buying a Pcar again, but the RMR and the label is such a draw
FWIW, I would never buy a 1999 with 20k miles. That is too low - I would almost be more comfortable if it had 40K worry free miles. The 20k milage on a '99 isn't worth the premium at all