Whether you build or buy my bottom line advice is don't over-analyze it. It's an AR-15 not rocket science. 16-18" barrel, mid-length or rifle length gas respectively, decent smooth hand guard, and definitely a good trigger. Barrel and trigger will be your biggest helpers. Adjustable this and lightened that...eh, not really a big deal to me. Military grade, 1/2 MOA this and "The Spec Ops Flavor of the Day uses it" that...who cares, that's not what you're doing. Save money where you can and spend where you should. Then get a ton of XM-193 or similar 55gr bulk ammo and go have a blast...pun intended. This sport is way too fun to get hung up on the trend of the day. My vote is make shooting your backside off and having SAFE fun the trend of the day.
And to second the comments above...welcome to 3-Gun...it's a great time!