The Hp is a standalone system. But you can plum and wire your Factory gauges into your Hemi. I just don't know how accurate that would be. Love you build and at least it's not LS.
Have you thought about a Holley HP system for the wiring? It'll save yourself a heck of a lot of headaches. It would make it a lot more user-friendly you could tune it with a laptop or Buy digital dash if you wanted to spend the extra dough.
for 4k you could build new motor unless your a good at fab work I would stay away form the 5.7 or 6.1 my vote would be a 416 but they are not cheap I have 10k in mine.