The e-28 is pretty rare I think, you could get some nice $$ for that.
The e28 M5 is pretty rare, too much fun to part with.
There are a few 535s, 528s around still.
My ragged two door daily driver e30 M3 is even more rare and does cost for upkeep, but it is a total joy to run and it gets damn good gas mileage when it is kept tuned properly. My first two months of having it, I went through a set of tires, warped a brake rotor and had to rebuild the transmission.
The 5 series M is garaged, waxed, detailed, mechanically pampered like an OBG/GYN examining the queen of England - - only to be treated like a whore under very special conditions, and boy, what a brazen slut she is!