MaicoDoug, I recognize your screen name from the E39 sub-forum on bimmerforums.
I would not buy a 200k miles X5 and hope to put another 200k miles on it. By no mean to say that it's not possible, but it will need lots of maintenance and parts/components replaced. It's better to pay a little more for a lower mileage one. The I6 has the same engine (M54) as your E39 530, not the M52 as you mention.
I would NOT buy that particular 4.4i you're looking at.
I would look for an I6: better mpg (as you want) no air suspension problem, easier to maintained... and a MT if you can find one. Driving 120 miles hway a day is quite healthy for the engine.
Good luck!

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00 E39 DINAN S1 //M5 (82k miles)
06 E53 DINAN 4.8iS, born 2/18/2006 (126k miles) http://www.xoutpost.com/bmw-sav-foru...s-my-dslr.html
90 Straman Z32 TwinTurbo Convertible, (1 of 44 ever made) 5 speed, 444rwhp/451rwtq
01 360 Novitec Spider F1 (26k miles)