Well, the Flagship was done over a
long period of time (better part of a year), using top-notch parts from many different sources--the stock and internals are LMT; the trigger is a Geissele SSA; the upper receiver group, including bolt carrier group and charging handle, are BCM; the handguards are Knights Armament; the scope is an ACOG.
90% of stripped lower receivers are just fine. Sometimes, you'll pay more for a name like LMT, Noveske, or whoever; other times, you'll want to stay away from names like Hesse or Vulcan Armament. Colt is known for making theirs out-of-common-spec, so I've stayed away from them, too. However, DTI, Mega Machine, Yankee Hill, Spike's Tactical, Stag, Rock River, CMMG, Bushmaster, Smith and Wesson, Fulton Armory, etc. are just fine--you get to pick what graphic is on the magazine well.
Billet lowers and upper/lower matched sets aren't needed for what you intend, so worry about those later.
A site like Del-Ton sells "rifle kits" that are everything you need to turn a stripped lower into a ready-to-shoot rifle, with the upper half already built that start (including shipping) for just under $500. Instructions can be found here:
http://www.wonderhowto.com/how-to-build-stripped-ar-15-rifle-lower-assembly-255593/
The greatest thing about the AR platform is that as time and money allow, you can always easily change or upgrade parts as you like. Heck, my first AR stayed in a basic configuration (everything stock, fixed rear sight) for a couple of years, and it never bothered me.
Internals, for your intended use, can be standard-grade, including the trigger. Hand guards, grip, and stock can be whatever you like--best thing to do is to go to a well-supplied gun shop or well-tabled gun show and handle a lot of different configurations to see what fits you best. Some like Ergo grips, some like MagPul, some like the basic A2--there's no wrong answer here. Get what you like.
As far as recommended starting points go, the only thing I would insist upon is getting a flat-top upper as opposed to one with a fixed carrying handle. If you want a handle later, you can buy a detachable one, but if you get a fixed one and want to add optics later, your options are severely restricted.
Magazines are an important aspect, too. The best to go with are MagPul PMAGs, Lancer L5s, or USGI magazines with MagPul followers. The USGIs are the least expensive at ~$9 apiece, with the PMAGs and L5s running about $15 and are a little more robust. Plus, some think the L5s or PMAGs look cooler.
Ammo. Be sure to save some money for ammo. Lots of ammo. More than you think you'll need, because chances are you'll head to the range with all your magazines loaded and a couple hundred extra, and wonder how you went through all that so fast.
Bayonet lug. Some places still don't allow them, but if it's not an issue for you, then get whichever you prefer.
Possible, but you'd still save money and gain knowledge by building exactly that with a bag of loose parts and the video above.
And you'd still need a rear sight, too...