Well if you take the 540, a lot of components are directly interchangeable between the two as far as body and interior, but most suspension components wont fit. The 540 has the added weight of the v8, so it has a different suspension set up.
It could get really expensive to get a 540 with 200k km and try to bring it back up to like-new condition.
I faced this same problem a while ago (swap engine or what?) and decided its best to either just sell mine and get a 540 or live with the weak little m20.