Gents,
These are old cars. Period. Parts as far as I know are not made of adamantium. So at one point they will break. Simple as that. Be it a Porsche or a Toyota. Everything breaks. The difference is how long between parts breaking and the price of such parts.
Another thing that plays a BIG role with any car is proactive vs reactive maintenance. The first one you replace things based on mileage, your feeling and so on, before something breaks. May seem like a waste of money BUT it minimizes the chances of getting stranded in the middle of nowhere AND avoids major things breaking due to other minor ones going down the drain (i.e. doing IMS bearing every time you do your clutch helps avoiding engine destruction).
The second case you wait until something goes wrong. You may save money assuming bigger things do not break with the small ones and if you are lucky and it breaks a block from your house you are not going to get stranded anywhere.
My approach with my 986 was to buy little things every once in a while and leave them at home and replace them when time came (mileage wise) or when replacing something else (so I would not have to open the whole thing again to change another part). This has worked very well as nothing major (other than the stupid piece of metal stuck between valves that required an engine teardown) happened and when the job had to be done I had the parts already, so no major price shock. That helps a LOT. One thing is spending $150 a month. Another one is $1800 in one shot after 12 months (even though the monthly amount is the same).
Also a car has a limited amount of parts. After you replaced most of them they will only break again after so many years.
Seriously, my V12 BMW had its injectors replaced a couple months ago at $120 per injector. But I know I am not doing that again for another 120,000 miles. Even the Boxster, after all the work done on the engine (new rings, new water pump, thermostat, IMS, clutch, etc) should be bullet proof for a minimum of 60,000 miles. So at one point after all the repairs things will settle. You are just catching up with the lack (in 90% of the cases) of proactive maintenance. Simple as that.
Regarding BMWs they can be rock solid IF and a BIG IF, maintained properly. My 2006 X3 is approaching 60,000 miles and has been perfect. Only minor work done. My 1999 BMW 750iL is now at 125,000 miles. No major work needed/done. But I keep an eye on all the cars and as I said, take a very proactive approach maintaining them.
I would say keep the car as not many things will go wrong now I bet. If you sell for sure the next owner will be happy as you took care of most of the needed things for him. LOL.
CR