GO FOR THE 20's. I got 19 x 8.5 in front with a 225/40/19 in front and staggered 19 x 9.5 with a 265/30/19 in back. The back sticks out a little bit and was hitting the fender but I added twist in spring spacers and the problem was solved.
I was originally looking at buying 20's but it was hard to come by a 20 x 7.5 rim so that the back tire wouldn't rub. I was the same as you though and wanted the staggered look so I found the Hamman pg-2 style wheel in chrome and I am happy with them. You will have to make sacrifies if you put the staggered wheels on a e 46 that is not the M3 because it will be likely to rub. Luckily there is also alot of stuff out there that will prevent this. Mine was a $10 set of spring spacers from Pep Boys
Ohh and the max offset would be +35mm in the back with a 9.5 wide rim. If you do go with and up to a 9.5, the tire I would recommend would be a 265 max width and a 30 max height.