Trading my 22"s for 18" M3 wheels?

Hey guys I recently bought my car and it came with 22" HRE wheels. I am not into 22" wheels and they make the car ride a little rough.. especially because I have the sport package.. Anyway, I put them on CL saying I would trade for stock.. someone just called me and said he would trade me 2005 M3 wheels with good rubber on rear and he will buy two new matching tires for front.. I told him I have the sport package that came with 19" and he said it's no problem you just put some slightly larger tires and it fills the gap or something.. I don't know if brakes or anything would be a problem going down to 18".. what would you guys do? Do 18" look funny on these cars? Any thoughts would be appreciated thanks!