well...the M3's are great..all of them...the only thing is that with anything 95 or above on the M3's is that it comes with OBD 2. If you are planning on doing a lot of mods with your car, OBD2 is a little harder to work with. There arnt that many upgrades available. I would say dont get the 318...cuz the engine sux, plus i hate the hatchback look of the TI. THe newer 325XI are good because they have all weel drive...which is really awsome, however...i think they only come in 4 door. I have a 99 M3, which i just got recently, and i love it.
BTW, you could do a OBD conversion on the M3's however i dont really know anything about it.
the only major problem to look for in M3's is the LSD (Limited Slip Differental). People who have them usually drive rather hard, and that is something to watch out for when buying one.
Good luck..let us know what you decide.