Do not even consider buying ANY 928 that is not at least $17, 500...and more towards $22,000.
Take your time.
CHECK each potential nice car with PORSCHE. Serviced properly?
Check that anything that has broken has been FIXED right.
Timing Belt every 30-45,000 miles,
Oil pressure?,
Tranny (Auto is much preferred and faster than stick in most situations),
Tires,
wheels,
Climate Control, and
AIR CONDITIONER REALLY work??????????IF NOT.......WALK AWAY.
WALK AWAY IF THE ABOVE ARE NOT OKAY.
And of course make sure the car hasn't been in a 'hard' accident, if it hasbeen then RUN AWAY.