It should be based on what your end use will be. If it's a 5.56 berm blaster, just buy off the rack. If you have a specific gun in mind that isn't generally available, then you build.
Start with an assessment of what target at what range? If it's paper, stick with cheap 5.56 under 500m, or consider an alternate if longer. You base the build on how far the bullet has to go, and how much power it has to carry to strike the target effectively (or ethically.) Get the cartridge defined for the job the gun will do 85% of the time, it will be the optimal choice.
That sets the barrel length, which sets the gas, which sets the handguard, then the trigger. After that, you add a flattop to connect to a lower to connect to a stock. And that is pretty much the priority of importance in the build. Too many build them backwards starting with the roll mark and working their way back to the flash hider. It's typical, and backwards. The gun is a launching tube for the bullet, focus on that and the optics first.
Here's where everybody details what they built (often in excruciating detail.) I wanted a deer rifle for broken woodland hunting in the Ozarks, max range is down a field maybe to 250m. I built a 6.8SPC with ARP recon barrel, nitrided, matching headspaced bolt and carrier. All the rest is window dressing, an AGP lower with trigger set screw, an LAR blem upper. I'm changing the furniture right now, a free float is in the mail. It's a lot of money spent for looks, it was an A1 with rifle handguard. It will be a MFT Minimalist with Apex Gator Grip. And, it absolutely will not be black, isn't now and won't be.
Biggest decision up front, will you build milspec with a pinned FSB, or go clamp-on gas block? I went clamp-on, I will have no trouble taking the barrel down and won't need to cut any delta rings or springs, which I can use on another rifle. It will likely take about 30 minutes to swap on the freefloat - not hours drilling out the pins and leaving ugly channels in the barrel to cover up. The military did that for a reason, if you are doing combat exits out a hatch, pin it. If not, don't hurt yourself later.
Other than that, read the assembly stickies at arfcom, and don't feel like you need to buy any special tools. They are just exactly that - common mechanics tools can and will do the job if you are building it to USE it, not store it as a safe queen. Worrying about getting a scratch in it is wasted effort if it's going to fall over in the hunting zone parking lot on grade rock in the early morning . . .