No, not all E53 X5s have the rear air suspension. It differs depending on the model year, and trim. For some models, it came with the sport package. For others, it comes as standard equipment even if there is no sports package.

Specifically, on the POST-facelift models (2004 onwards): all 4.4i and 4.8is models came with self-levelling rear air suspension as standard, even with no sports package.

For the post-facelift 3.0, it was not standard. However it automatically got included in the 3.0 with sports package. It was also available as a stand-alone option on the 3.0.

FWIW, I don't think you need to go out of your way to avoid the air suspension. I know some folks have had issues with it, but I have had zero problems with mine.