Oh GOD, I love those old G-class 4x4's...they're second only to 'Mogs in terms of off-road capability!! The ultimate irony is that the modern-day G500's (you know, the $100K plus SUV's) are basically this vehicle with some minor body changes and a new interior. The new ones are, from what I can tell, just as good off-road as the originals...just, they cost so much that you probably wouldn't want to take them off-road.
A non-turbo 617 was actually a factory option in the G-class. In fact, I think this is the first g@$$er G-class I've seen; most of them are 300GD's. I'm not sure if you could ever get a turbo 617 in these from the factory or not, but if a non-turbo will bolt in, I don't see any reason why a turbo wouldn't go in. If what I've heard is correct, you shouldn't even have a problem with engine mounts or bellhousing patterns...apparently, M-B kept everything standard. I found that out when I was researching the possibility of putting a BOMBed OM617 into an R107 body.
It might be difficult to find a "proper" clutch assembly, due to these not having been sold in the U.S....but, while it might not be "right" per se, I don't see any reason why a 240D clutch wouldn't work...