Consider the difference in an M3 vs a non-M3, springs, shocks, brakes, calipers, engine, some cases transmission, body parts (varies), dash guages, interiro trim..., it could be a big deal, ragardless it would be less expensive to just buy an M version of what it is you want.
With an e30 the big expense is going to be the engine, to rebuild one is $$$! I have seen a few nice e30 M3's for $10k or less lately.