you should be able to run up to a 28 mm offset with no problem if you stick with the recommended 18" tire sizes. If you choose a tire size thats wider or taller than recommended...28mm might be too high.
All BMW 5/6/7/8/Z8 wheels are interchangable and have the same "basic" wheel specs (except the E39 5 series):
- 10mm-28mm: wheel offset range (the new 5/6/7 series have a few staggered wheel set-ups that have rear wheels as high as 32mm)
- 72.56mm: wheel hub (EXCEPT the e39 5 series which is the oddball with a 74.1mm wheel hub)
- 5x120: wheel bolt pattern