re: What do I need to know about Ar Lowers purchases then Completing the gun?

Yeah, so ... I'm in the Flagship PSA store on Fernandina about once a week ... picking up parts, buying ammo, picking up online orders, etc.

If you live near a PSA store you can save transfer fees, and sometimes shipping costs, by shipping directly to their stores.

If not ... if you live where there are no stores, then you'll have both lowers shipped to your favorite FFL and, hopefully, they'll only charge you one transfer fee if they're on the up-and-up.

Get your lowers and then order parts that fit your specific needs. I've built a bunch of ARs, 10s and 15s, using PSA lowers. They have a great $29.95 sale going-on right now for their blem lowers.