Engine mounts are the easy bit. Its all the electrics, connectors, senors and other related bits that are going to make it a loooooong and frustrating project (even if you go the 325 - then you have springs etc to swap). Stick with 'vehicle original' parts.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i