If the 50i water pump is anything like the 48i, be warned they fail constantly, are a pain to replace, and the hoses and other parts are very expensive. Of the problems to worry about, the water pump would be at the bottom of my list. The N55 doesn't use the same water pump as the N52 (328xi) but it's a similar model. The N52 one seems to fail more frequently for some reason, but I'd take it over the N63 mechanical one.

The bigger issue on the N63 are all the hoses in the hot-v and its propensity to grenade. That and the V8s are not pleasant to service in general, so valve cover leaks, valve stem seals, valley pan (not sure how that works on the N63), etc are all a royal pain. I prefer the engine note and power, but HATE servicing the V8s.