The SPCII chamber with .100 leade and 45 degree shoulder is the latest improvement. Barrel twist of 1 in 11 or slower allows the use of tactical ammo, or simply lowers pressure up to 5K from the peak.
Other than getting a bolt headspaced for that barrel, all the rest is based on common sense parts for your build. The particular optimum setup is a 16" midlength, because it will reach out to 400m. A longer barrel doesn't add much to power or accuracy, just length and bulkiness for the user. That doesn't mean other lengths won't work. The cartridge was based on a 14.5" barrel, and 16" is already longer - plus legal for over the counter sale.
It's a medium range hunting cartridge - it's meant to hit harder than 5.56 and cover the same ranges. With that in mind, low power scopes or red dots on flattop uppers fit the bill. It's a tactical or hunting caliber, making the gun tactical or hunting, meaning it shouldn't be black. Pro users don't use black guns, they camo them, and have since before the M16 was type classified. It was OD and grey in the first years.
Since most 6.8 shooters aren't wearing body armor, stancing squared up in CQC drills, use a fixed stock. And since even the principal spokespeople of KAC admit the average shooter doesn't need 48" of rail, like on the M4, use a tube with bolt on strips. Or avoid the expense with issue type handguards.
That's one way to look at it, with connected progressive application for what it does to a specific purpose. Everything contributes to a specific use.