dont get smaller wheels to lower it. the 20's were meant for the size of the challenger wheel wells. itll look like crap with 18s.

lowering it on the stock shocks will be bouncy eventually,usualy u can get away with a 1 inch drop without hurting anything. anything more than 1 inch u will want shocks meant for lowering, or coilovers (which im not sure about the suspension on the challenger are available.)