My water pump snapped off (2006 Boxster owned since 2014 and 90000km) on Hwy 115 half way through a CTMP track weekend at about 145000 km. There was warning though — seepage around the pump. The irony was that I had a new pump scheduled to be installed the following week.
Another common failure is the AOS (I had mine fail filling the air with oil smoke at 141000 km). There was warning there as well (a little too much smoke on start up) — should have heeded and replaced the AOS – the part is relatively inexpensive for a Porsche part.
I have also had a manifold header develop a crack in an inaccessible place (it was cut, repaired and rewelded at the cut at about 95000km and failed again at 150000km – so I had a used pair installed). I don’t think that is a common problem though.
I just this weekend had a gas tank vent line/solenoid replaced that was causing an intermittent CEL but again I don’t think that is a common failure.
Besides those failures everything else I have spent money on was discretionary (new wheels, upgrade stereo) and regular consumables -light bulbs, tires, brake pads, rotors and sensors and regular fluid changes (brake fluid every spring, oil every fall). I go through tires, brakes fairly often (I have been on track 6 times a year for the last 4 or 5 years).
Mine is on the hoist 6 times a year for track inspection so it gets a pretty thorough going over often.