Re: 18" rim
kenntona;416999 said:
Stock/OEM offsets are always conservative - you will NEVER be fender flushed, unless you use spacers.
I found this to be true, unfortunately. Stock rims have a high offset, so they tend to look recessed. This is one of the "flaws" they partially rectified in the facelift E90. I hope they fully correct it in the next 3 series.
Stock 17 x 8J rims (+34 offset) will look less recessed on your widened FL axle, but will might be about 20 mm short of being flush with the fender. As Kentonna mentioned, some use spacers to extend the rim outwards. I personally feel uncomfortable about putting anything between the axle and the wheel.
The 17 x 8J stock rims will protrude outwards by half an inch more compared to the 16 x 7J's wheels that come with the 320, so 17" wheels won't look so badly recessed. Observe E90's on the road which have 17" stock wheels and you might find it OK. Hurry had a brand new set of 17 x 8J stock 158 rims a couple of months ago, might still be there.
I feel there is no middle ground. One must make a clear choice when choosing a wheel and suspension option. One either commits to the "comfort and fuel efficiency" philosophy and stick with the cushiony stock suspension plus 17" or 16" wheels, OR pursue the sporty approach which means committing to lowered springs, firmer shocks and 18" or larger wheels. If one blends sports with comfort, the results tend to look and feel incongruous.
If it is 100% important for the wheel to be flush HORIZONTALLY with the fender, chances are you will eventually want it to be flush VERTICALLY, ie have a narrower vertical wheel gap. Then, you might prefer sports approach: get a 18 or 19" rim with positive offset of between 32 to 35, or order DPE rims custom-made to fit the FL E90 perfectly. A coilover could give a moderate drop of 30 cm to match the big rims. The ride will not be as smooth and serene as a stock car, but the drive will be more exciting, providing your passengers don't complain.