Just about any new AR-15 that you’ll find should be reliable after break-in if not sooner.
Keep it well cleaned and lubricated but swabbed somewhat dry after lubing for storage (to avoid dust sticking to the innards much), and clean, lube and swab it dry after each use. Just before going out to fire, some rifles like more lube than others (a little wet), especially before break-in. In case of jamming during break-in, make sure that you know the procedure for un-jamming it before going out.
The easiest way is to buy a complete rifle and have it sent to your FFL (an ATF-licensed firearms store in your state). The next easiest way is to buy a *complete* upper receiver and *complete* lower to go with it. The lower will need to be sent to your FFL (lower receiver being legally designated as a firearm).
As for putting more of it together yourself with parts and instructions, anything with a *complete* upper in your kit should be good and easy to put together with instructions and proper tools (like punches for putting lower receive parts in, doable without with good instructions but much easier with).
But if you go the more complicated route with mounting your own barrel on an upper receiver, make sure that you know what you’re doing in advance—every detail. You’ll need a vice block, a barrel nut wrench, a torque wrench, a bench vice and *good instructions.* Some good instructions might still be posted to YouTube (haven’t looked there lately, myself). Other sites also have good video instructions. Some of those sites are firearms vendor sites.
If the barrel nut is for a fancy floating hand guard, it might require a special barrel nut wrench. If it’s a standard barrel nut, then you’ll want a wrench like this one...
https://www.jsesurplus.com/precisionreflexar15barrelnutwrench.aspx
...’cause it won’t slip like most of the other ones do.
And know the max torque for putting a flash hider on a barrel before doing that yourself. If you over-torque it, it can stretch the rifling a little at the end of the barrel and cause some inaccuracy. The Vortex flash hiders tend to be much pricier, but they only require hand-tightening and torque themselves afterwards. Easy...
https://en.wikipedia.org/wiki/Vortex_Flash_Hider
[Disclaimer: I’m not a gunsmith, and I’m not a lawyer. Anyone with good knowledge feel free to make suggestions or offer other ways of doing things.]
Good stuff,,
Thanks