I think Nix should stay at this point for one more round of stocking up on depth and to figure out what head coach would fit the personnel that he has already put into place. If Chan goes then Fitz can go with him because the team won't need a QB who needs to know Chan's system. Change Defensive Coordinator, too, but bring in someone who will run primarily a 4-3 because who wants to make that switch again.
As for finding a head coach, I can't imagine that it would be so difficult as most here have described. Going into the season, a lot of people had the Bills as a playoff contender and the pieces are almost in place. There are obvious limits in the decisionmaking every game and any good coach could see this team as only a good coach away.