if i were you i would consider 18s over the 20, and i say so because in the 18 if you go staggered the rear wheels have a much bigger lip and if your staying stock height the car wont sit so high. as far as tires im not sure what kind of roads you have in WI but i wouldnt do anthing shorter than a 40 series and width 245 up front and 275 rear.
out here in ca a staggered set 18x8/9.5 with tires will run you just under 900 out the door. when i bought mine i paid 600 for just the wheels and they were 18/20 staggered. if you can get the rears 9.5s for the same doesnt seam so bad to me.