No problem. By US wheel, do you mean the difference between bolts and nuts/acorns?
Bolt
Acorn/Nut
Offset explained in a nutshell...
The offset, measured in millimeters, can be negative or positive, and is the distance from the mounting surface to the rim's true centerline. A positive offset means the wheel is front of the mounting surface, closer to the center of the fender; a negative offset means the wheel is away from the mounting surface and projecting from the fender. Offset affects the scrub radius of the steering and it is advisable to stay within the limits allowed by the vehicle manufacturer. However, if tires are fitted which are significantly wider than those specified by the manufacturer, a compromise may have to be adopted whereby a wheel with less offset is used to prevent the tire rubbing on the suspension. (thanks Wiki)
See here for more information on wheels for your car:
http://bmwe34.net/e34main/upgrade/wheels_upgrade.htm
In general, anything 19" and under is acceptable. Standard sizing (non-staggard, that is... wider rears than fronts)
225/55-16, 235/45-17, 235/40-18