I'm in pretty much a similar situation - my e90 xi came with 16's which I can't stand so I'm looking at 18's or 19's. I'll probably end up with 18's, but regarding the ride height, there have been pics posted where xi cars with 19" wheels on stock suspension didn't look that bad at all. A bit of fender gap but still very decent.
Check this thread for pics:
http://www.e90post.com/forums/showth...=665540&page=3
Remember that all the pics there are of xi cars, which ride higher. Also it seems like on stock suspension, with 18's/19's, fender gap is larger on e92's than e90's. Not sure what yours is.
If you want the look without sacrificing the ride, then the only real solution is coilovers - unfortunately, it is also the most expensive. C'est la vie.
Wheels first for me, I'll live with whatever fender gap I get for a few months and enjoy the stock ride until I can afford KW V1's.
0.02.