Your best bet if you need a gun "right now" is to buy a complete gun. Parting a gun together is very difficult right now as parts are very scarce.
If you want to build a gun then the most important things are to get a good barrel and a good bolt carrier group. I'd suggest using quality parts from Colt, Daniel Defense, Bravo Company, and Noveske for these important parts. A lower receiver could be just about anything. There are a few odd-ball lowers out there that are known to be out of spec or junk but for the most part a lower is a lower. Lower parts kits are a place to watch out for junk. Higher end kits like Colt, Daniel Defense, Stag and CMT are usually prefered. CMMG is usually considered a lower end kit but I had very good luck with the one LPK I've used from them. Actually ended up having better fit and a better trigger then my Daniel Defense kit. I've heard of people having good luck with PSA also.