The best way to level is with the Bilstein 5100 adjustable struts. They are much better than the standard spacer type leveling kit IMO and will give you a better ride quality. They have height settings from 0" (stock height) to 2.8" of lift. Contact the guys at Bully's Performance for pricing on the Bilsteins at firstname.lastname@example.org
. Be sure to mention that you're a forum member for discounted pricing.
I'm assuming you meant a +1mm offset, not 1". 20x9 +1 offset with 305/55/20's will fit on there no problem. You'll probably be sticking out about 3/4" past the fender so the fender flares will cover them up.