Just bought a 99 740i---stock (not a Sport--couldn't find one price right) I want to upgrade to 18" wheels and to stiffer suspension. Does a simple spring up grade do the trick? (or is it a waste of $ and time----do I need new struts etc also to do it right?). My car is certified to 100K mi---would such upgrade/s effect warranty coverage Thanks in advance for any advice--insight Dave S