Except for Horford, and maybe IT, I'd trade most of the rest of their team and the picks for Westbrook and Griffin or Cousins. That would give them a core they could build around. The rest of the team would be what's left after the trades. Right now, I doubt they can get much beyond 50 wins.
As for player options, I think with that kind of a winning core, probably a 60 win team, the players would want to remain with the Celtics. The key is to get the two stars.