This is what I did on my 335 for less than $200:
http://www.e90post.com/forums/showthread.php?t=86651
I think you could do something about the same on your car. Tire Rack is always willing to let you know if the a certain wheel and tire setup will meet the requirements for a spare if the wheel/tire combo is not the same size, but the circumfernce matches.
I have a conti pump/repair kit for when I am in town, and I throw the spare in the trunk for long drives.