Coming from a Dinan 540 you will want the 4.6is! The milage is certainly worse than the 4.4i but the power is fantastic!

Find one with up to date maintenance. I have replaced the transmission, Mass Air Flor sensor and various front suspension bits to the tune of about $6,000. Get an aftermarket warranty!!

Take your time and try and find one with some of the upgrades you may want.

Spend some time reading and searching the forum to learn about the various issues and modifications for the X5.