Even though I didn't pay for it (was saving up for something better), one of my first cars I drove around was a 1990 535i with 150,000 on the clock. It was owned by a family friend and was purchased new in 1989.
All I can say is from the bullet proof reliability, to the LSD, comfortable like new interior, and the overall smoothness and performance the cars are just sweet.
I auto crossed this car, lived on the skidpad at the driving school I worked at and just flat out drove the ever living piss out of it and it never once burped or had any issues. My family friend currently owns it and it's going on 180,000 strong with only minor wear and tear issues.
If I had to choose one now I would without a doubt get a 1995 BMW 525i with the M50 motor. With simple obd1 tuning via T.R.M and some $300 schrick cams you can see 200-210 whp and you won't have to live with the 15-24 mpg of the 535i