If you're going to build it...
Get the Complete PSA (Palmetto State Armory) 16" M4 Flattop Upper Assembly for $399, and the PSA MOE (includes MOE buttstock, MOE trigger guard, and MOE pistol grip) Lower Build Kit for $124.99 (ar15.com Group Buy). You can find a MOE first Gen. rear site for less than $40, or the MOE Gen.II rear site for less than $60.
Get a legal lower from a local Gun Shop fpr $65-$99. If you are willing to hassle finding a local FFL willing to accept a transfer, you can get a PSA lower receiver from PSA for about $65 right now.
The scariest part about buying parts is not knowing where they were made, and how they were made. all parts llok good sitting in the box when you receive them. You'll find out more about them when you try to fit them (too loose or too tight) into/onto the weapon, and after you start shooting it (does it function, and how is it wearing). The cheap rifle parts kit that I bought in a kit from a predecessor of a large major company needed some parts to be filed to fit correctly, the bolt was not assembled correctly causing an intermittent FTE (failure to extract), and then some of the parts wore rapidly (primarily the hammer) after the weapon was assembled and used.
Buying an assembled weapon with a lifetime warranty doesn't do you much good unless you don't mind eating the the downtime, the cost of shipping, and the shipping risk. I haven;t had any parts failures in over 20 years of owning ARs, and if I was to fins a part that needed replacement during an after range time session, I would simply replace it from my stash of spare parts I've been accumulating over the years. With the exception of major parts that may need gunsmithing, it's easier to replace the defective or worn parts yourself.