Turbos are generally mounted high so oil can flow down to a level above the level of oil in the sump. Twin turbos might turn out to be the easier way to go. I don't know if the firing order supports twin turbos or if it even matters.

Since it's obviously going to be an involved task, consider swapping in a late 70s M100 (6.9) or early 90s M119 (5.0). Check out this website:

http://www.mbcoupes.com/memberscars/T/satishtummala/satishtummala.htm.

Sixto
91 300SE
81 300SD