The cost up front may be lower, however the extra time trying to get everything to fit and get the wheelbase correct and the ride height and stance right IMO would outweigh the cost saving. Going to a used frame isn't only just labor, the used components would probably need to replaced anyway, rotors, calipers, suspension bushings, etc. If the original frame is not bent, rusted, or cracked it will work fine. You may want to weld in some boxing plates when you install the new suspension.

Coming from a "been there, done that" mindset, I doubt your savings with a used frame would be little if any over the cost of buying an IFS kit and installing a late model rear end.