Man this is such a huge topic. You might check out Arfcom (AR15.com). It's pretty clique-ish and there are a lot of brand fanbois but there's good info as well.
My thoughts:
Accuracy lives in the barrel.
Reliability lives in the BCG.
You'll never regret the nicest trigger you can afford. Corollary: triggers are an easy upgrade, so you can start with a GI trigger and get a nicer one later
There's no reason not to include a free-float forend or adjustable gas block on your build.
You will learn more by assembling your upper from parts than buying a completed upper.
Buy an "oops kit" with extra pins, detents, and springs. Detents and springs come from outer space, and will launch themselves into orbit if you give them half a chance. Once they're gone, they're gone.
Billet and "bling" lowers, and clever engraving, look nice on Instagram - but you can't see the decorations when you're shooting it, and $$ spent on cosmetics is $$ that didn't get spent on a nicer barrel or trigger, or ammo.
Stake everything that needs to be staked.
Longer gas systems are more pleasant to shoot. An 18" mid-length or 20" rifle-length system is a very smooth shooter.
You can save money on your lower, handguard, muzzle brake, gas system, pins & springs kit, buttstock, and furniture. If you cheap out on a no-name or store-brand barrel or BCG you risk being disappointed until you replace them with what you should have bought the first time around.
You don't have to spend too much on an upper, but you might want to pass on it if you don't know who made the cheapest upper on the shelf.
Think deliberately on what you want to do with this rifle. Just a range toy? Get an entry-level 16" or 18" barrel and a blister-pac parts kit from your LGS and spend your money on ammo. Contemplating some serious predator calling? Consider a medium-heavy 20"-ish barrel, nicest you can afford, carbon-fiber free-float handguard.
Then there's the question of calibers. Thinking of hunting something bigger than coyotes, from a saddle? Check out the 350 Legend, 6.8 SPC, 6.5 Grendel, 6x45, etc. Do you reload, and is there part of you that just has to be different? Check out the AR Wildcats at Mad Dog Weapon Systems (
https://maddogweapons.com/) (I don't work for them, but I've bought from them in the past).
Palmetto is good. So are CMMG, BCM, DPMS, CMC, Geissle, Noveske, Faxon, Aero Precision, Alex Pro, Matrtix, Strike Industries, MI, and others. Brownell's doesn't sell anything that's not worth having, but they might not have the lowest prices on the 'net. Midway's store brand, AR-Stoner, is good'nuff, but you can get a nicer barrel for a little more money.
Avoid disreputable or no-name vendors, at least until you're savvy enough to recognize brands you trust and brands you don't. If a vendor advertises on Arfcom, you can probably trust them to not sell you crap, but they'll probably mark up their prices to cover their marketing budget.
Considering the current politics and supply chain issues, prices and availability might be frustrating.
Good luck, and post pictures!