A “cheap” older base model Challenger has zero value in all of the stuff you’ll be removing. Engine, trans, diff, axles, brakes, wheels, interior pieces. By the time you recover the value of all that out of your Hellcat, you’ll have the same entry cost either way. The Hellcat is going to have some accessories and stuff like covers that add up fast if you don’t have them to transfer over. Starting with a older (15/16) Hellcat may actually be the most cost effective way to do it.