first off welcome to BMW-forums
a know of a few people who have swapped various engines into the e30's. depending on what model engine your refering to is also a huge factor if anything go with an s50 or s52 (1995 and 1996-1998 m3 engines) ,as mentioned above it takes money, the s50 and s52 are both more resonable as far as swapping goes, there is less work in the electronic deparment and the engines are smaller than the s54 (e46 m3 engine). another consideration to take into granted is when swapping the s54, there is lots of extra electronic things to be weary of such as the fly-by-wire throttle, complicated sensors etc etc.. extensive engine bay work has to be done to clear room for the s54, everything from motor mounts to moving accessories around and cutting for clearance. on top of all this motor work your also looking at a suspension upgrade to handle the extra weight, brakes, it is quite expensive to do all this work ( ive really shortened this whole post down i appologize , its just there is so much involved) in my very personal opinion id take that money and look into forced induction.
for what its worth i think the e30 M3 is perfect byitself, its balance and power makes it the best M3 of all of them (thats my opinion)