tinsnips, a drill press and a grade-8 bolt with a 1" shank, plus a hammer, an anvil, and somebody else's garage door.
You'll also need some hogwire to make springs with.
openstreetmaps.de (or .org) pretty good tools, no tie-in with Alphabet (alphabet soup? really? they can't just call themselves the glow-in-the-darks and let us run over them in the night?)
having built four ARs with parts from disparate vendors, I'd say that for as much as everyone tells you to test everything every build … it's not likely to go wrong unless you're doing something a little bit weird. 5.45 bolts, for instance. There are two standards, and most of the barrel manufacturers know whose bolts will work, and whose won't.
Side charging ARs have similar problems. Bear Creek puts the handle in one place, everyone else somewhere else. So, don't use a non-BC side charging upper if you bought a BC side charge or dual-charge carrier.