(Disclaimer: I have never purchased aftermarket wheels myself. )
1. Yes you can put other brands of wheels on your car. I don't know if there are any kind of warranty issues this would void.
2. The performance issue is mostly related to the weight/size of the wheel, not brand. Bigger wheels have lower profile tires and this helps with fast cornering. However as the wheels get bigger, they tend to get heavier and this decreases acceleration, gas mileage, etc. Of course the other issues include asthetics (bigger wheels will probably look better on this bigger 3-series) and durability (more trouble with potholes, etc).
3. As to what type you should consider, I would look at the size of the wheels that would have come with the car if you had ordered the sport-package and mimic those. The car has been engineered to perform best with those sizes. Even if you order non-BMW wheels, I would get those sizes that would have come with the car in sport-package.
My personal preference if I was doing this would be to buy the BMW wheels, but that's because I don't have experience with others and I trust BMW. From what you've said, even if you order these later, you avoid the 100% tax.
All of that said, I'm sure there are others on this board can give you other input regarding aftermarket wheels.
Whatever you choose, congrats on your new car!
fiesta said:
thanks for sharing your thoughts. This last moment I dont want to modify my order to prevent any delays. so I guess i ll be getting a wheel cap

I ll see what can I do when i get the car. Either put a 16 inch to not buy new tires or buy 18inch with new tyres depending on the prices and my ability to sell the current ones.
One last question...
Can I put another brand wheels on the car?
Does it have any bad effect, like decreasing performance or corruption on engine or mechanics?
I ve heard that there are different type of wheels. What type of a wheel should I consideer?
Thank you...