A few good builders I've seen other 6.8 buyers on this forum recommend: LMT, Noveske, AR15performance.com.
As for configuration of the rifle, if this is an all-purpose rifle in 6.8, you shouldn't need to go longer than a 16" barrel. The 6.8 does well ballistically out of shorter barrels and going longer won't gain you much. Also, you have a good idea of setting up 2 rifles: one in 6.8, and a similar config in 5.56 for affordable training. Given the cost of quality 7.62 rifles these days, you could nearly set up your 2 rifles in 6.8 and 5.56 for the cost of ONE rifle in 7.62.