I'm not an expert and don't have a ton of experience. I have some experience and have done a lot of reading and formed my own opinions. The 3 gun crowd will tell you to build a different rifle than the crowd that depend on their rifles to fight for their lives. The 3 gun crowd will probably lean towards a fast and match accurate AR and the defense crowd probably a reliable and combat accurate AR. Nobody is wrong here, they just want rifles for different purposes. You can take my advice with a grain of salt if you like.
From what I'm getting from you, you want a general purpose AR. It's your first so you'll probably not only want to be able to shoot matches with it but also use it for defense, range plinking, training, etc.
If it were me, I'd build a 16" midlength for those purposes. Stripped receivers are very hard to find right now so I'd likely buy what I could find. Aero Precision makes receivers for a lot of companies and the general consensus is that they are good to go. I used a PSA lower (made by AP) and a surplus FN upper on my last build. Both work fine. Barrel and BCG are the most important, in my opinion. FN, Daniel Defense, and White Oak Armament are barrels I'd look at. PSA sells FN barrels. I used a Green Mountain barrel recently because it's what I could find in stock at the time. Haven't gotten to really put it through its paces yet. For BCGs, BCM, Daniel Defense and LMT are what I'd consider based on their reliability and having Carpenter 158 bolts that are HP/MP tested and shot peened. I don't think RRA shot peens or HP tests their bolts and only sample MP tests. I've also heard good things about PSA "Premium" BCGs that are assembled with a Carpenter 158 bolt. For LPKs, I've just been using PSA's kits. There's better quality out there but they were cheap and in stock. I've heard great things about the LPKs from G&R Tactical. Kits are pieced together with some of the best parts. I've just been using milspec triggers but that's something that can easily be upgraded later. The milspec trigger that came in the last PSA LPK I used is a beast! It's very gritty and not really conducive to accuracy. I hope it will smooth out over time but I'll likely upgrade it.
From there, you can add a stock, receiver extension, buffer, spring of your choice and handguard/rail and sighting system.
