What year car you putting this in?
You don't need a new alternator and can use whatever is on the car now for a VR (what year is your car?)
The Magnums all came with a "mini" starter unless you don't get one with the engine. Mid 90's Dodge RAM / Dakota 5.2 / 5.9 starter
Manifolds, headers, fine
You need whatever you'd use in an LA for a distributor for carb. "Keep the EFI" (thinks I)
Mags don't have a mech pump. Easy answer.........electric, rear mount pump
I know nothing of the serpentine belts, but I bought a 96-7 junker RAM with a froze up AC pump. someone put a shorter belt on there and bypassed the bad compressor, so that might be one way.
Consider keeping the EFI. There are ways to do factory, or use Megasquirt, and others.