My Challenger was set up with power disks from the factory, not a bad set up. Nothing wrong with manual brakes either though, and if you've got a big cam or are planning a rebuild with one you might consider staying manual, since you may not have the vacuum to run the power brakes.
With 14" wheels you'll pretty much only be able to run the stock size brakes, 10.87" will be as big as you can go. And if you're doing that I would just find the parts instead of buying a whole kit, but that's just me. Anything larger than that will require an upgrade to 15" (or larger) rims.
Switching over to the Mopar disks is pretty easy, you just need set of later spindles, caliper brackets, calipers, brake hoses, a disk/drum master and a new proportioning valves. Calipers, hoses and the master cylinder are all available at places like RockAuto or your local Napa, I wouldn't even mess with used stuff for those. Spindles are pretty easy to find, unlike what that article says you CAN use F/J/M body spindles. The change the geometry slightly because they're taller, but it's actually a more beneficial than negative change. I was just at the local wrecking yard yesterday, there were several donor cars that spindles and 10.87" brakes could have come off. It's actually pretty easy, with a pickle fork, hammer and a couple of wrenches you can have everything from the spindle out in half an hour. Spindles can also be purchased new now if you don't want to mess with it.
Only thing I can't remember is if the '70-'72 e-bodies used smaller upper ball joints. If they did, I know Doctor Diff makes a set of sleeves to use the original UCA's and ball joints with later spindles.
I've got a couple of set of "slider" type caliper brackets for the 10.87" brakes too if you need a set, I'm in the process of upgrading all of my cars to the 11.75" rotor.