Anything can be done with enough time and money. No one has dropped a late M3 engine in a six series as of yet. Turbo charging the M30 engine is the best way to go. As for the strength of the parts, the differential, 265 transmission, driveshaft, and most of the engine are stock in a car that makes 800 hp at the crank. BMW has always overbuilt drivetrain items which is why you commonly see 400k mile bmws still running. As for the new engine vs. old engine debate, think about this. BMW in the 70's heat cycled the 1.5 ltr f1 engine hundreds of time to give it strength. This is the same process that you do to your engine everytime you start and stop it. This cycle dramatically increases the strength of the metal and is then known as a seasoned block. That engine turbocharged produced 1300hp, strong indeed. I try to find old M30 engines for the same reason. The only way to produce reliable horsepower is with a standalone system(haltech, Mototec, etc). It is the key to building high horsepower as you can control and adjust every conceivable aspect of an engine. Sixes handle like a heavy sedan and are by no means on rails. That said with enough suspension mods they can be adequate, more of a high speed cruiser than a twisties car.