wheel size i recommend using square setup and supplement with spacers as you prefer. square will net you two things. you eliminate some of the understeer and makes the car more neutral. secondly you can rotate tires front to back extending usable life of your tires and saving some money. on the e90 i have been using 275's front and back on a 19" wheel. why 19". simply because i like how they look. 18" work fine and little cheaper on tire costs. i went with a 3 piece forged wheel from forge line. its race proven and very strong wheel. little heavier but definitely can take a beating.
"It gave you amazing satisfaction, but anyone who says he loved it is either a liar or he wasn't going fast enough." - Jackie Stewart on racing at the Nurburgring