I am converting my original front drums to disc brakes. Will a 1970 set of spindles and arms and shields from a Duster fit into a 1971 Demon? Can I then use 4.5in bolt circle wheels? For a car that will see very limited strip and mostly street time, who makes the best disc brakes and rotors for the buck? I would like something towards the top end but dont want to break the bank. I mean what is something for calipers, pads, etc. that is better then say an over the counter store like NAPA??

You need 73+ abody upper arms and balljoints, spindles, calipers, rotors, shields, proportioning valve, master cylinder, hoses etc. To get the 4.5 inch bolt pattern.

The 1970 spindles will have small bolt pattern hubs on them, if original.

and yes, the '70 spindles will fit your '71
For street use, the OEM stuff is good but harder and harder to find. You must get hubs. None of the aftermarket rotors come with the hubs (although way way way back parts stores sold hub/rotor sets).
Rebuilt calipers should be pretty reasonabley priced from parts stores. Shop around. Core charge may be high if you don't have a pair to give them.
For new rotors, the Centrix (sp?) seem to be above average.
For street use, might want to look into Raysbestos's products. I'm personal running the Chrysler dealer replacement pads so haven't had to experiment much there. Some of this is personal preference - that is, how you like the brake pedal to feel. Some of that will also depend on master cylinder, rear drum cyclinders and adjustment, and proportioning valve set up.

You will need either a combination distribution block/proportioning valve/warning light switch for disk brakes from your donor car.
or
Add a proportioning valve in line. This is what the factory did (I think through '69 ). You can also buy an aftermarket adjustable prop valve pretty cheap. Hard part here is making good double flares in the brake tubing. Often its better (safer) to use preflared tube sections from your local auto parts store. A little creativity in making stress releiving bends and loops will get everything to fit.