Final update . . Well, I guess this is it. I put in the new expansion tank, and sensors, but nothing has changed. Then I pulled the spark plugs out, and every one of them looked like this:
Major oil fouling. The cylinders were even worse. Just looking into the spark plug hole, I could see the oil glistening on the top of the pistons. This poor Bimmer is fucked, could be worn piston rings, valve stem guides or seals. And there ain't no way I'm gonna tear the engine apart and rebuild it, just way too much already, and neither do I have the time on my hands.
It's not a definite decision yet, but I'm pretty sure you'll all be seeing parts from my car in the classifieds section