Realize that you are getting advice from a guy biased to shooting High Power (Sterling Shooter), so some of his advice might not be in concert with your stated goals.
Based your follow up answers, I'd probably push you towards 18" with rifle gas. 16" with mid-gas will be okay, but a tad more wear and not as soft on the recoil. 69s are fine to 600 yards and the suppressor does not care about 69 vs 77s. 1/7 or 1/8 twist would be acceptable. The gas system is something you need to really think about. Most adjustable gas is not, they are set and seal, or you will have failures. The Noveske switchblock is good, and while I have not personally tried them, the SLR appears to be robust enough as well. Other than that, go fixed and set up for suppressed and consider a buffer swap for un-suppressed.