If I was buying an AR for use as a duty rifle (LEO or contractor in Iraq) I could see spending $2K.
But for the average Joe, who wants a reliable and fun AR, primarily for range use and as a home defense (SHTF) rifle, then any $600 to $800 AR would do fine.
If you have the doe and want high quality, Colt ARs are a real deal these days.
Take the rest of you money and buy a 1,000 round case of ammo and after you've shot that up, you'll know the platform well and be in a much better position to make informed choices about rifle mods. And even if you aren't comfortable assembling a lower, anyone who ever played with Lego blocks can swop uppers, stocks, charging handles, sights, optics, etc.... That's the whole 'modular design" aspect of the AR.
I'm a big advocate of buying firearms in the caliber they were initially designed for. 7.62x39 AKs, .45 acp 1911s, 9x19 BHPs, and 5.56x45 ARs, as the entire platform was designed around those parameters. After your well familiar with the AR in it's original chambering, you'll better understand the strengths and weaknesses of the cartridge.
If you absolutely must spend that kind of money on your first AR to make yourself happy, buy a flat top colt and then drop a spiffy high end optic on it.
imo, 300 Blackout is a reloaders cartridge... you won't shoot a rifle chambered in 300 Blackout a whole lot if you don't reload.