Hi mike,
I dunno about the technical requirments of fitting but I've seen two e39's (both 530i's) with e46 M3 18's. It looks kinda weird. I don't know if they are copies?
They looked like the real thing (from driving past)?
I've also seen an e46 with e39 M5 18's. Now THAT looks strange.
Personally I don't like the 18" M3 wheels. The 19's on the other hand, luuuuverly...........
good luck,
MrGrant