Colt's lower is a future proposition, a response to FN offering a kit to run 5.56 in the 17. If you like the heft and weight of a .308 rifle shooting an intermediate cartridge.
You can buy a lower online, they just need an FFL to ship to. That FFL should want to get it, and you should be ok with his fees - all of that agreeable up front. Some sell other makers lowers, if they have one on the shelf for $130, and you want to get one in costing $60, expect the silent treatment. They very well may not want you as a "customer," AR builders are snakes beating them out of their well deserved profit.
Once the lower is in hand, everything else can be shipped direct to your door, subject to state and local laws. And there are some stupid ones.
Well liked lowers are those that get talked up because fanboys like to promote what they bought as a social pecking order. Basically, most of them are made by about a dozen machine shops and forges, who do them all on contract. Pretty much a commodity item, pick a rollmark and pay whatever surcharge you can afford because of it. Good lowers and uppers can be had from $48 up. Really. Buy a Noveske upper, they even gave away a blem lower.
If anything, choose exactly what you plan to do with it, and build to that, not whatever is the maker of the month. Start with barrel and caliber, then upper to fit what optic, furniture, and last, trigger.
In 6.8SPC the better suppliers are ARP, Bison, White Oak, and others. That forum - 68forums.com - is the best source of 6.8 specific parts and details. You will be treated fairly there, members shoot all the alternative calibers and have a balanced opinion. One thing to be aware of is that any caliber other than military surplus is commercial priced - none of it is bulk packed for 25C a round. That takes a government selling off it's outdated surplus, or ammo makers dumping overruns and reject on the market. You get what you pay for, 6.8SPC is an American hunting caliber, loads are well made ammo, not cheap steel cased underloaded fodder for punching holes in dirt.
I'm finishing my 6.8 build for this falls hunting season, plenty of suppliers and ammo online, Academy even sells Remington 115 OTM. Not the hottest ammo out there, I doubt a deer at 75 yards could tell the difference. I haven't bought one special tool to assemble it, and it all came to my front door - less the stripped lower. Check the value of the ARP bolt and barrel combo, it's money ahead if you are building. I got it with the milspec and nitrided BCG, the combination was cheaper than parting out all of it and paying multiple shipping and handling.
