he is in US so im guessing he's refering to the US spec M3? because down there they call it S50 for 3.0L and s52 for 3.2L M3 motors...
nice ride, awesome in fact..
you need a bunch of conversion parts either way you go..i'll start you off from what i can remember..
E30 IX brake booster
E34 oil pan
E28 Master cylinder (this and the E30 one might be mixed up)
i suggest you get the M motor mounts, i think you need an E34 M5 mount and some other mount (off memory..which sucks)
and a few other parts...if you get the S52 you need OBD1 conversion parts:
OBD1 intake manifold
OBD1 wiring harness
i think theres something more, and this can all be taken off an E36 325 or 3.0L M3...also if you get a S52 i suggest getting an 3.0L M3 or 325 throttle body because the non ACS-T throttle bodies flow better. good luck