That’s the beauty of the AR-15. Completely modular. Anything from a 16” M4 clone, to a specific hi-power build 20’HBAR. The sky’s (or your wallet depending) the limit.
I like the KISS approach. A2 upper, 20” HBAR, 1:9twist, 3-point sling, and 20 round magazines.
The advantage if you build it is, chances are you’ll be more familiar with the inner-workings and be more adept at recognizing and replacing if it stops running.
Fair warning though, for some, assembling can be addicting. They don’t call it Black Rifle Disease for nothing.