Skier1: X5SND will give you good advice since he has those wheels on his E53. You are correct about the hub-bored size and offset.
For the 21", the rears would be 21"x11.5" offset38. I think you can get by without the "iS" flares. But it would look better with.

BimmerBlueblood: I don't think weight is a problem. The hub-bored size, offset, width will pretty much determine if the wheel will fit assuming the bolt pattern is 5x120. Good thing is the E53 has the same hub-board as most bimmers (72.56mm) The E39 and E70/71 (fronts) have strange hub-bored at 74.1mm
Why would you want to put 18" wheel on the X5? Most of them already come with. If yours has 17" wheels, skip the 18" and go to 20".

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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)