First thing to do is find out if any BMW approved makers do a lowering kit, such as Alpina or AC Schnitzer.

I put AC Schnitzer lowering suspension on my 2006 X5 and this was fitted by BMW and remains under full BMW warranty. It was a bit more but got rid of the warranty issue. Also try to find an authorised ACS dealer not just your BMW dealer to give you prices I saved a lot going to an authorised dealer rather than BMW although they fitted it. Check and see if your BMW dealer has a or can get a copy of the official BMW/ACS catologue. Also try and chip him on the price he quotes you should get at least 10%

Here is the link to ACS X3 catalogue direct.

http://www.acschnitzer.com/englisch/...tt_acs3_x3.pdf

Hope that helps