I own Unique Customs Wheels & Tires (
www.unique-customs.com) and also have a E53. You can run staggered 18" with stock wheels but its not recommended at all. You should run at least a 9" wheel with 285/50R18's to avoid problems with possible leakage of air, and your stock wheels with this wide tire will give the tire side wall a roll effect in turns and sharp curves which could cause the bead to break free. Its not a definite thing that will happen but be aware of the chances of it happening causing you more trouble than its worth.