Hi
Just going through the same thing myself at the moment, mine is a 99 328 sedan but I believe they are the same (or at least close). I have 3 speakers in each front door, one in each rear door and 2 in the rear parcel tray. The bad news is the speakers are non standard sizes, and low depth. I believe the speakers in the lower part of the doow skins are approximately 5 1/2" as are the ones in the parcel tray. There are tweeters in the frint corner of the doors but I read that the mounting is a little odd, and not very welcoming to aftermarket parts.
The options I know of are:
1, shell out for Bavarian soundwerks direct replacements from the states, this will be expensive but will work without any modifications, plug and play.
2, make adapter rings for 5 1/4" speakers but shop around for similar depth speakers. I believe Alpine and a couple of others even come standard with adapters that might suit.
3, custom work, I don't think it's worth it depending on what you are wanting.
Good luck, read around a lot, there is quite a lot of info on line.
Bruceda