From what I see, the 745i would be about the same cost wise as the X, so wouldn't be saving much money... A friend of mine bought a lease return 750i from wholesale auction for $25k-$28k and hasn't had any problems with it yet a year or so later, but when they have problems that are different than what the X5 would have it is usually electronic and control modules are expensive.
I personally would prefer an E60 550i if I had the choice of cars to get, maybe that one is in my future, post X5...

(not anytime soon though)