I'm very opionated and am picky about my AR parts but for a good reason. AR money comes far and few in b/w so I have to make each penny count.
That said... Innovative Tactical Systems LLC, Made in USA, AR 15 uppers, piston operated tactical upper, 5.56, 6.8spc, custom precision barrels
is well respected in the 6.8 world. Noveske is is the cats meow. DD is more than it's chalked up to be. LMT has a great reputation.
BCG - No experience w/ LMT BCGs but a lot of guys use them. Don't let the MIM talk scare you away. DD and BCM make great BCGs too.
There are a lot of rails I'd go w/ before YHM. DD makes the best rails period. Troy makes a great rail I like a lot and have purchased multiple times. The Samson EVo I just recieved is very sturdy and Centurion Arms makes a quad rail I really want to own.
IMO hobbyist and SHTF don't mix. One has UTG and a cheap RDS the later has a lot of thought and quality parts thrown its way.
Have you thought about the cost to feed a 6.8? It's not cheap.