If you on the edge of the decision, how about drive 3.0 X5 to compare, it is not a rocket launcher like 4.4 but M54 is pretty hard working engine. I am not surprised that you're after X5. Just few month ago I sold my 2006 X5 3.0 6MT and bought 2009 Range Rover Sport SC and miss crisp BMW handling. What I am trying to say that 3.0 is not a lazy car, depends on expectations of course but as far as 3.0, there is reasonable short list of issues like (I will list from biggest pain in the butt to easy DIY):
Oil Filter Housing Gasket
CCV Valve kit
Valve cover Gasket
Beyond that, there are few more things and if you handy enough and willing - should not be such a big deal. At least engine and most of its components accessible/reachable in 3.0 VS 4.4 and yes... Valve stems in 4.4 are heart (but mostly wallet) breakers
Keep in mind that X fives are not all created equal, some have regular spring suspension, some have airsprings in the back and some equipped with air ride on all 4 corners. There is a standard seats/package, comfort seats package and sport package but most likely you know most of that since you had mentioned that you're not new to own BMW