A lot of people are using
Palmetto State Armory stripped uppers & stripped lowers. There are a lot of different brands out there. Prices are not constant. In other words some companies are cheaper at times.
Barrels are a preference and their quality & performance differs.
The lower parts you use could be considered more of a preferences. I honestly haven't used all that many different brands, but so far I like Rock River lower parts.
Trigger type/brand is also a preference with function as they are not all the same. They all function, but if you've ever tried a high performance trigger you'll know what I mean about the difference.
Hand guard type is also a preference, but it is true that accuracy is best with a free float hand guard. The MOE furniture is a drop-in replacement for other plastic non-free-float hand guards.
Sights are a preference.
FRONT SIGHTS: Many prefer a fixed front sight. Others prefer to use folding front sight. Folding front sights can be attached to a free float hand guard and other folding sights can be attached to a railed gas block.
REAR SIGHTS: Many that like to use optics prefer to have folding rear BUIS (back up iron sight). Some prefer a fixed rear sight.
Whether or not you choose to fully assemble your own AR from scratch is up to you. Price it out and compare what you would get by buying a fully assembled upper.