And you heard correct about BCA's quality. You have have about a 70-75 percent chance of getting a good centerfire upper and a 50/50 change on their rimfire uppers.
I am going to make a suggestion that you either wait for better complete uppers or better parts are on sale. This is especially true when it comes to an upper that will be capable of shooting very small groups. I would not want to hamper the accuracy of 6 ARC or even 6.5 Grendel by going too cheap on the barrel. You don't have to buy top tier stuff but you also don't want the bottom of the barrel stuff either. I would suggest looking at Faxon or Odin Works barrels if you are wanting to build your own upper and are on a tighter budget. Since the 6 ARC is a very accurate caliber, it begs to have a better trigger than a GI trigger. The Larue MBT-2S is hard to beat for the money.
The 6 ARC uses the same bolt as the 6.5 Grendel. And some people get away with using 6.5 Grendel mags with 6 ARC while others do not. I have found that the ASC brand of magazines work well and are available with the correct 6 ARC followers. I know that ASC doesn't have the best reputation when it comes to 5.56 magazines but all of their 6.8 PSC and 6 ARC magazines I have work very well.
For your needs a 16" barrel will suit your needs. Now if you later decide to start shooting the 6 ARC at long distances then an 18" or longer barrel is better. The 6 ARC is very capable of shooting small groups.
I am running an Odin Works 18" barrel with +2 rifle length gas. The Odin Works barrels comes with an adjustable gas block, though I haven't had to adjust mine.
Here is my 6 ARC build
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And here is how the rifle shot with both 105 and 108 grain ammo
108Gr - Green and blue was getting it sighted in and yellow circle is a 5 shot group at 100 yards.
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105Gr - Yellow circle was a 5 shot group at 100 yards
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