Spikes Tactical has served me well through the years but Wilson Combat will always be my favorite. And they may offer a discount if you let them know what the purchase is intended for.

I've never handled a Wilson but have heard nothing but great things about them. I personally am a huge Daniel Defense fan.

When you buy, I might consider a 300 BO over the 5.56. Should be less likely to over penetrate which I would think would be a good thing if God forbid you ever had to use it in that environment.

Sounds like a great program. Need to see if our school has one like it. I'd do that.

The whole "you aren't an armorer" is primo fuddlore. It's not like he's making an 80% lower with misaligned pin holes and poor sear engagement.

If it was my pick, I'd build on an 11.5-13.7 upper receiver. Short enough to be a little handier, while still staying above the 2400fps threshold for cavitation. Something like the Aero 12.5 would be a solid budget choice, and you can bump up to BCM, SOLGW, etc if you want a little more premium materials. https://www.primaryarms.com/aero-precision-m4e1-threaded-barrel-upper-556-carbine-atlas-r-one-12.5

The other advantage of the 13.7 is that with a pinned/welded muzzle device you can save the $200 and trouble of registering it with the ATF. So that $200 can go towards a solid optic/magnifier setup like an Eotech and G33 with a good flip to side mount. The Holosun setup is a decent budget alternative to that and will save around $500.

Buy a complete gun, do not modify it in “any” way, never ever use reloads. I can tell you from first hand experience; you ain’t got the money for what these attorneys are going to do to you.

Seemingly lost in the ruckus is that the OP was wanting a complete gun in the first place. There are usually a few for sale on the forum at less than $1k. He didn’t say he wanted to build one.