What transmission do you plan to use? If you're going manual or old auto, Mopar I believe has a plug and play for the 6.4L. If you're going newer auto, Hotwire Auto can do a harness and let you know what parts you'll need from the donor vehicle. Within reason, I would get everything you can from the donor vehicle including the gas pedal, transmission shifter(if auto), and any and all computers. As to which engine to use, that's up to you and what you can afford. The 5.7L would be the cheapest way to go as it was used in more vehicles and can be a little older. I'm using a 6.1L as I found a good deal on it. As daryld stated in his thread, things add up quickly.
Corey