I don't know if all dealers are like this, but my dealer said BMW would not order the car with different wheels, I HAD to have it with the wheels it came with, so if your going to buy another pair in addition to your regular wheels(like I did) that's gonna run you some $$$. Your best bet is to get a 325 with the sport package, the 17's look good. I don't even think you can buy a 3 series(m excluded) with an 18 inch wheel option from the factory, unless your dealer tells you different. At any rate, wheels bigger than 17 would be expensive. The 17's on my 330xi look good, I think you'll be happy with 17.
Titsilver metallic 2004 330 Xi 6speed manual