Question about rim size

[armensti]

i was thinking of changing either to 18" or 20" but i cant figure out what offset wheels to look for. i want a thick tire set up like 305 in the rear and 275 in the front. i know if i go 18 it would be possible but i dont know if i can find any rim that will clear the sport package brakes. what offset do i need to get the 18s? and if i get 20s will i be able to fit 275/305's. i know 20s would clear the brakes no problem but what offset do i need for it. also i dont want to use spacers and i dont want to sacrifice handling. would it make my life easier if i went with 20s? i think i wouldnt sacrifice the handling because i would be able to get something with almost the same offset if not the same.