my buddy's company, YKW motorsports, sells a 550i sport package replica wheel in 18" called the BMR-1 that will fit your car (http://www.ykwmotorsports.com/euro_bmr1.aspx
They tried fitting a 17" version beforehand, it fit, but there was only 1mm of clearance! The 18 fits much better.
Visit Cross Ave Auto (www.crossavenueauto.com
). They carry YKW stuff.
Talk to Sam, say Derek from YKW sent you.