Honestly, to make it a quicker and easier build keep it as Mopar (and simple) as possible.
It's a "crash bumper" Dart, not one of the '68-70's which are the highest demand ones, so a stock restoration isn't sane. They can have that tough muscle car look with steelies and dog dishes that not every car can pull off.
Best bang for the buck, don't do the Hemi for the Challenge. A 360 Maggie with a 4bbl intake (either redrill the heads for the LA bolt pattern or look around for deals on a Magnum bolt pattern) and you have a 300 horse motor bone stock. It's a roller cam setup stock. Not much used available in performance rollers, but they are out there.
360 LA ('72-92) motor would be cheaper, but have lower compression. One from a '90-ish truck would have better heads than the earlier ones, and lots of used speed parts available.
A 904 V8 trans can be had for cheap. 727's a little more but good deals are out there. Balance weights on the converter would need to match the style of 360 in front of it. Not a biggie.
Rears, the 8.8 Ford is a tough one to beat. They seem to be everywhere like 9" rears were 20 years ago. The 8" Ford rear is probably the cheapest route to go, and they do hold up. Mustang/Maverick ones are the right track width. Different centers are available used. Rears seem to be pretty cheap when guys are selling them off to put a 9" in there.
I love my 8 3/4, but getting one under the Dart at a good price with decent gears takes years of being a Mopar pack rat like me.
Yes, it can be made to handle with mostly stock parts. Moog offset upper control arm bushings are cheap and easy. Addco made sway bars for that car for decades so used ones are out there. The mounting hardware stinks, but fabbing up something better to mount it isn't hard at all.
The big disk brakes are off a late '70's Cordoba/Magnum. They were on the other late B-bodies (Fury/Monaco, but not the C-body Gran Fury/Royal Monaco the yard was telling you about), and R-body St. Regis/Newport. Think T.J. Hooker squad cars. You'll need the caliper adapters & rotors from the donor car. They could be found with either pin or slider calipers. The M-body Diplomat, '82-up Gran Fury & 5th Ave have the same rotors you have now. That is if it already has disks. If it doesn't, grab the spindles from the donor car too.
A similar Dart was built by a buddy and his step son. They stuffed a Comp hydraulic roller "truck" cam (biggest under .525" lift) in a stock Maggie 360, put on a Chinese knockoff of the Ed RPM manifold (dirt cheap), a used carb of some sort, stock Mopar electronic distributor, cheap used headers, and a 3.00 8" Ford rear. Car was turning 13's with traction problems. And I know they had less than Challenge money in it.