I think with the pre 2001 vehicles, they have the round pin connector, and later ones have standard ISO connectors. You can get a loom adapter quite easily though if you have the BMW type, which will give you the standard ISO connectors that you can use on any modern day stereo.
If you have audio controls on your steering wheel, you'll almost certainly loose that functionality. But again there are adapters can get it back for you. The adapter is specific to the manufacturer of stereo you use. Some high end stereos have the ability to be programmed to accept the resistor inputs of the steering wheel, so it maybe possible to do this without needing the audio control adapter. In any event any stereo will work fine without the steering wheel control "luxury".
I put a Pioneer FM/DAB in mine. Works fine, and sound is much improved. Even on the original speakers.