Buying a stripped lower and parts kit will probably not result in saving money, especially when you add shipping costs, but some people like to learn how to put their rifles together.
I recommend that you buy a complete lower receiver (including pistol grip and stock) from your dealer, then ordering an upper, either from Bushmaster or from www.gunsmokeinc.com
You won't be happy with the 20" HBAR though, as it weighs a pound more than the M16A2. Instead, order a 20" A2 "Government Profile"
barrel from Gunsmoke, which is only available threaded, and have them remove the bayo lug and permanently install their A2 muzzle brake. This barrel is turned down thin under the handguards, exactly as the M16A2 is. THAT will get you 99% the same as the M16A2.
HBARs only have any advantage when made from a match-quality barrel blank, or (with standard barrels) when used on full-auto machine guns.