As for the BMW 6, I was into the euro car thing for quite a while and I never heard of anyone having problems with one (aside form owner induced, no oil, burst radiator hose and kept driving, etc.). So I would expect with the typical German (the Bavarians do it too) overbuild it should have plenty of meat to hotrod.
If you are looking for a factory hot rod (that maybe you could get on the cheap) the Audi inline 5 turbo always had lots of low end grunt, and being turbo you should be able to pump up the power fairly easily. My mom had two of those and the early 80's 5000T's had crummy turbo's (changed twice in 120K mi), which you wouldn't care if you were swapping it (should get it cheaper if it has a bad turbo). Of course if you were looking for a HUGE project you could swap in one from a Quattro, with the whole drive train, All Wheel Drive anyone?
