Just buy a complete rifle.
Davidson Defense = NO
Delta Team Tactical = NO
Bear Creek Arsenal = NO
Republic Firearms = NO
Anything else = sure..
Buy once, cry once. Get quality parts the first time around. You'll be glad you did. Trust me. I learned the hard way.
I have a couple decades experience of building ARs (to include the Army's armorer course) and much prefer building my own. I like Stag or Aero Precision for upper & lower receivers & BCG, Ballistic Advantage for barrels (Hanson profile is awesome & comes with a pinned gas block), DSG for lower parts kit, MagPul for stock & grip, and Aero Precision for hand guard.
Mil-Spec as in, Cheapest product available that just barely meets contractual agreement?!
Yeah. I understand. All of my AR's are built by me on 80% lowers.
Put your money into a quality trigger group, and a quality barrel.
If you're only going to build one, it's not worth it. Too much of an investment in tools. Buy a built rifle.
If you plan on building more, buy a barrel extension fixture to hold everything while you wrench on the barrel. https://www.amazon.com/Wheeler-Engineering-One-Piece-Construction-Gunsmithing/dp/B07MFR1685