I would say the engine and tranny are one of the most reliable BMW has build, next to the M30 that is! The M70/72 were pretty bullitproof, and so is the 5HP30. Far more reliable than any V8 they have made, but they have electrical problems. Mostly with DME's (that end up burning the coil), and throttle bodies that are $2000 each new! Millage does not seem to affect them in any way, in the last few years i had 4 of them (in 750il's) from 120K to 300K+ and they all perform the same!
Like most older HIGH end BMW's, this is not something to do on a budget, unless you DIY, or at least do a lot of research and know people to work on it for a reasonable price. If you bring it to a random shop everytime something goes wrong, thing will get expensive FAST!