You lucky dog! Why would you want to change those awesome 16" Sport Evo wheels?? Really though, with the Sport Evo you should be able to fit 18 inchers much better than the average E30 M3. The Sport Evo has larger front fenders exactly for that reason. I still however would recommend going with 17" wheels. The performance increase (in handling) gained by running 18" wheels is not worth the harsher ride and greater chance of scuffing your wheels (in my opinion). I also think 17" just looks better. Check out this link to help you find good tire sizes for different wheel sizes.