I've got to have another dig at this thread. First thing to eliminate is the upholstery - it suffers from the normal 116 leather problems. The rear cushion has collapsed, and the driver's seat can do with quite a bit of TLC - in other words, refurbishment. Also, someone should align the fitting of the front bumper bar.
That's enough of the criticism. What about the rest of the car ? The principle of the conversion is very appealing. Contrary to BenzBoy's assertion, I still regard the car as a W116 Mercedes Benz S-Class. Never mind the engine - well, in fact, if anything, it needs to be considered an improvement. It begs the question - why do we have a 116. Looks, desirability, quality of manufacturing ? If I had a 450 (well, I do, but forget that), I'd have no hesitation in installing a more modern, up to specification engine. It would add to the desire to owning a car like that.
On the other hand, if it came to replacing an M100 engine in the 116, the answer would be an emphatic "NO". That would be tantamount to killing the car. A 6.9 is a 6.9 and should always be that. They have their own issues suspension wise (perhaps), but even that feature I would want to maintain and it is the engine that makes it special enough to want to own and cherish one. Never mind that there is faster, and probably better around, but it's still a 6.9.
So you need to ask yourself why you have a 116 - is it for the engine, is it for the really appealing '70s retro look with all that chrome ? Therein may be your answer. I further think that condition is of great importance - I have seen some 450s, 280s etc. in immaculate condition both body and interior wise, and there is no way that one would want to fit mechanicals from another manufacturer. Just my thoughts. Regards Styria