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I've "built" (assembled a lower and then attached a purchased, complete upper) a couple but I'm certainly no expert. I can second the recommendation for Bravo Company USA, I used their mil-spec buffer tube kit and have been very happy with it. The difference between mil-spec buffer tubes and commercial buffer tubes, and the related collapsible stock fitment, is something you might want to look into.

I used Palmetto State Armory for both of my uppers and probably a few random parts I don't recall. Their shipping is pretty slow, but otherwise I highly recommend them both for quality and prices. Most of their barrels (possibly entire uppers, I don't recall for sure) are manufactured by FN, chrome lined and cold hammer forged, on the same equipment making military contract rifles, they just put the Palmetto name on them.

I've heard the bolt carrier group referred to as the heart of an AR, I wouldn't scrimp here. I used Palmetto BCGs, but Bravo Company also offers great ones. Things you may wish to look for in a BCG are full-auto/M16 profile (no, it won't make your AR full auto and there's no paperwork required to purchase - it just has a bit more mass to it, and some people think that makes them more durable and better functioning), high pressure tested (hpt) and mag particle inspected (mpi), a properly staked gas key (google pictures for this), and possibly a coating or plating on the carrier that will make cleaning easier later on.

For lower parts kits, I've used both CMMG and DPMS. I don't recall any quality differences between them, but for a first-time builder, I definitely recommend CMMG. Their parts kits are sorted in color coded bags to help you identify all the different little springs, pins, etc, and they have better installation instructions (including a great online installation video).

If you want to install back up iron sights that flip down, I'm very happy with the Troy Industries variety. I've read mixed reviews of the polymer Magpul variety, some say they don't hold zero very well, others say they work just fine.

I don't have any suggestions from personal experience for you on lower receivers. I used two really oddball receivers that I don't think you'd be able to easily find for sale, so I won't bother talking about them. I have read great things about Palmetto State's receivers, and I'll probably use one of theirs if I build another AR. I've heard that same thing about there only being 4 manufacturers of lowers and the rest being rebranded, but I suspect (with no evidence) that there are a lot more small machine shops producing lowers now. I know a local gun shop offers billet lowers in their name that I think are produced locally.

Also, if you want to support a Texas company, I've heard great things about LaRue Tactical, but I have no personal experience with their products.