Why not just buy some new steel, used auto parts cost money and are not known alloys so proper heat treatment will be by trial and error. Between your time and cash outlay you will be ahead of the game buying some new steel. That being said the best stuff are axles, leaf springs, coil springs, torsion bars and steering linkages. Keep in mind you may expend all of this effort only to get material that is unsuitable to forge what you want to make.