I agree with the others who said it, unless you have some very specific needs that can't be met, buy a complete upper. PSA mentioned above is one of my go-to's for those. It's not that building one is difficult to do. It is more that for a basic upper you can't build it for what you can buy it for. At least not if your time in tracking down and then assembling all of the parts is worth anything.
However, if you NEED it to have things like a Lantac BCG, A.R.M.'s folding sights, SI Latchless Charging Handle, FN America's chrome bore barrel, Apex Gator Grip hand guard, and a Yankee Hill black gas tube, then your only option is to build it. Just know that the BCG and hand guard together will likely cost more than a basic complete upper.
Even my entry level builds get an upgraded trigger. Nothing super trick, but a very nice trigger that is light years better than what comes in the typical complete LPK's. So I buy trigger-less LPK's and
ALG's "ACT" trigger. Brownell's used to, and may still, sell them bundled together.
I also like
BAD's Sabre Tube butt-stock. Here in the PRK telescoping butt-stocks are dangerous and our nannies won't allow us to play with them. Frankly the Sabre Tube is my default choice now even if I could buy a CAR buttstock assembly. Lighter and more rigid.
I bought both of the Sweeny books and Zedecker's book on building an AR. I liked all of them. The spiral binding of the Zedecker book lets it lay flat on the kitchen table.