The only advise I can give you, you already know. With BMW the latter in the series the better. However after saying that, I bought a "used" 97 528i. The only "problem" I have had with it so far has been a replacement of plugs after one bad morning of a very rough start. After the plugs replacement months ago, the car has been fine.
If purchasing older, I would do two things: 1) a hot oil flush and 2) a transmission flush - not the older type of "flush" which was really a drain and change, but a true flush. At Midas here in Vancouver, they are charging $150 for the trany flush, so it must be cheaper south of the border. The hot oil flush is $40.