I love assembling these rifles. It isn't actually building, just assembling.
I've assembled 20 or 30 since I started in 1996.
I still don't have Delrin receiver blocks. Or a vice.
Or special tools to install roll pins.
Or special tools for detent spring & pin sets.
I don't need them, but you make your own choice.
It's your money, not ours.
A good set of hands is indispensible.
Most kits come with a complete upper receiver, so you don't have to assemble the barrel to the receiver, nor install the flash hider. Might as well start with one of those kits, they're cheaper and totally complete with the right parts (you don't accidentally get M4 type ramps on the barrel extension but commercial ramps on the upper). Then you can disassemble and modify the rifle extensively over the next 10 years. Eventually you figure out what parts to buy (and not buy) to assemble then next rifle from scratch with all individual parts, in the configuration you want. Cool !!!
Tools you cannot live without? Maybe none.
If you get a collapsable buttstock, it takes a special wrench to tighten the nut to the lower receiver.
If you get the standard buttstock you need no special tool.
If you want to remove and install a barrel on the upper, it takes a special wrench on the barrel nut.
If your kit has an assembled upper, you need no special tool.
Everything else - EVERYTHING ELSE - I assemble with plain ordinary hand tools. No exceptions. You can spend your money on anything you want, it's your money. Not mine.
Headspace:
One of the fine design elements of the AR rifle is the use of a barrel extension between the barrel and upper receiver. If the barrel extension is installed correctly (get your barrel from a good reputable source) and the receiver and bolt carrier group are properly assembled, the headspace is generally set for you.
But you MUST go through the normal safety checks after reassembly. If you don't already have books or honest online resources, go find them. They will explain the procedures clearly.
If you assemble your own AR, it's up to you to learn how to do that safely.
It isn't like tying your shoes. Full and complete responsibility for safety is in your hands. Learn your skills first, then assemble.