First, welcome to the site. There is a lot of great information on this board.
I bought my first AR, a Bushmaster A2 20” HBAR, as a complete rifle. That will be the last complete AR I’ll ever buy. I’m a big believer in building the lowers and buying complete uppers. Building a lower is not difficult and it allows you to customize it. I have built RRA and Eagle Arms lowers and always use RRA lower parts kits (with the exception of some triggers like the Jewell). Most of my rifles are a mix of lower and upper receivers by different manufacturers.
For my Varminter I built a RRA lower using a RRA parts kit and a Jewell trigger. I’ve got the RRA NM trigger on another rifle and really like it. However, on this rifle I wanted a trigger I could adjust. I then bought a complete RRA 20” Varminter upper. Do a little research on barrel length and velocity and you’ll see there is a big gain in velocity between a 16” and a 20” barrel, but not as much of a gain going from a 20” to a 24”. I strongly agree with Wingman26 on the 20” barrel. These Varmint barrels are HEAVY. I love my RRA Varminter but would not enjoy carrying that in the filed for a hunt (well, unless I was just setting up somewhere and wasn’t going to move around a lot).
I went with the RRA Varmint upper over the Bushmaster because of the Wylde chamber and the 1:8 twist. I wanted the rifle for shooting heavier bullets and planned to use a lot of military ammo. Your needs might be different.
Stick with quality manufacturers and you’ll be fine. Most people recommend Armalite, Colt, Bushmaster and RRA. I personally think RRA is the best bang for your buck right now.
One more thing. Go to your hometown forum and introduce yourself to the other shooters in your area. It’s a great way to meet new folks and try their equipment! [:D]