My e36 320i is a heap of junk and suffers all kinds of problems, including the windows, door cards and ball joints mentioned
Engines are prone to overheating, mine has had headwork before my ownership. My car has suffered difficult to track down electrical gremlins all the time I've had it, ranging from a failed crank position sensor to indicators that occasionally cut out (Still can't find out why

).
I've enjoyed driving my car but in retrospect I'd rather have had a 318is (M42/M44), mostly just because the fuel economy is not as good as I want it to be. It is a nice fun car, a little heavy if you like things light and darty but certainly chuckable down the lanes.
I will probably be breaking the aforementioned car if you want any bits, two useful bits that come to mind would be a set of 40mm lowering springs with associated monroe dampers and reproduction motorsport alloys.