re: help on an AR-15 build

Well the link I just posted has what is called a freefloated barrel. Meaning nothing rests on the barrel besides where it joins the upper receiver. This is generally more of accurate setup than a polymer handguard like the link you posted, as the hanguard rests and gets support from the barrel. Also this setup puts you pretty far ahead of the game in terms of adding accesories to your foregrip later on down the road. With the mlok handguard you can buy accessories from many different companies that have attachment devices specifically designed for that handrail. Such as angled foregrips, bumpstops, light attachments.
It's generally been the tradition to opt for a freefloated handrail over the polymer handrail and it has been an increasing trend for that hand rail to be mlok.
If you don't think the stainless barrel is your style they have nitride barrel options as well and I won't get into the differences between the two because it normaly ends in heated debates but I will say for you either will perform to much higher standards than you will ever call on them.