I think the most important point is that you need as much of the M3's electrical as you can get. Dashboard, engine wiring harnes and ecu etc. Prefferably you want the car sitting there to pull all the pieces your going to need. If you don't you are going to spend a LOT at the dealer. From looking at the swaps a local shop has done this isn't "hard" it just requires you know what your doing.
Dave