We are doing a resto-mod style challenger in our shop right now. We are buying a 2013 5.7 hemi for it. They can be bought for 2500-3000 range for late model low mileage engines. The 09 and up have the large port eagle heads , and a 6.1 intake will bolt up (same port size). Hotwire and modern muscle sell the ecm, and wiring for these engines. Comp cams vvt camshaft in a 09 5.7 challenger made 454 horsepower at the wheels .I believe 09-up 5.7 engines are the best bang for the buck.
Dyno Testing Comp Cams Hemi Phaser 274 Camshaft for VVT 5.7L & 6.4L HEMI Engines: Challenger - YouTube