The only thing I had to cut off the car was the mount for the expansion tank. The front subframe had to be reconstructed. The rear subframe also had to be reconstructed to use a 2.65 diff from the original SL600 to be compatible with the 722.6 5 speed trans. That will not be required if you use a manual or an earlier 4 speed.
For electronics I ran the stock ME system as it was needed to talk to the 722,6 trans controller. Allot of problems go away if you loose the 722.6 transmission but the 5 speeds overdrive with lockup converter is a very nice thing to have.
All of this is pretty much documented in my build thread. See
http://www.peachparts.com/shopforum/performance-paddock/178997-m120-into-560sl-pictures.html
For custom bell housings see.
http://www.peachparts.com/shopforum/performance-paddock/271090-non-mb-trans-mb-engines.html