what wheels+tyre do i get?

Hi
I own a BMW635csi Highline (L6)
she is b-e-a-utiful.:screwy:

I`m after 20" wheels and suspension, i dont want to alter the arches.
Ive also considered 20" rear and 19" front.
Can any one of you guys help? will they fit for a start, ive seen some 635`s with 20" wheels and their arches looked standard
The offset on my current wheels are e20 my stud pattern is 120/5 i think. with 225/60/15 tyres.
What will this do to the handling? would a new anti roll bar be prudent as well, if you think so could someone point me in the right direction as i cant find anything out there.
What suspension kit will fit my car with 20" wheels?

cheers john