FIRST: I agree that
buying your first AR15 is the easiest and smartest way to go about it. You'll have factory support and less of a chance of an issue. You can always upgrade later. I say this because from your original post, it seems that you stated that you have little firearms experience. Building (think "assembling" more than "building") an AR15 is not hard, and I never bought a "pre-assembled" AR as I've built all mine. But then again, I've been involved with shooting sports since I was a kid. So for me, understanding the basics was not tough.
SECOND: If you decide that you want to build/assemble your first AR15, there are a couple ways about it. The
easiest way is to buy a "complete upper" and "complete lower" and snap them together. A
little harder than that is buying a "complete upper" and a "stripped lower" and a "lower parts kit", and assembling the lower. The
hardest is buying a "stripped lower" and piecing everything else.
When it comes to terminology of an AR, you have the "upper receiver" and "lower receiver"... basically the lower is easier to build than the upper.
THIRD: Find good resources to decide what route to do.
WATCH THIS VIDEO IF YOU ONLY WANT TO WATCH ONE VIDEO!!!!!!:
AR15nerd is a good blog/website that I've found for beginners. Here's a link of the basics on building and understanding the AR15 and its nuances. Again, this is entry level info.
https://ar15nerd.com/how-to-build-an-ar-15-the-basic-parts-you-need/
There are plenty of Youtube personalities going over AR15 builds. I'd check out Sooch00 and his build videos. He does a pretty good job explaining. Here are a few other videos.
The first is a super detailed video that I use to build. You can see if you are interested.
The second and third is the basics of the AR15 platform.
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FOURTH: Find good brands.
For buying a complete and basic rifle, look at brands such as Smith & Wesson, Ruger, DelTon, Aero Precision, Anderson, DPMS, and Palmetto State Armory. There are others but those are off the top of my head. Normally, a rifle will run about $400-600 depending on accessories and sale prices.
For buying a complete upper receiver and complete lower receiver and attaching them, look at Palmetto State Armory just to make it easy. There are always plenty of sales. Prices will be around the same.
AND REMEMBER, the lower receiver... whether it is stripped, complete, or a full rifle... must go through an "FFL" (federal firearms license... i.e. a gun store)