Before anything else you have to to something to signal the regime change. If it was me, sign a guy off the street right now with good NBA experience who don't want a lot of minutes and who can police that locker room. Where's Crash Wallace these days?
Beyond that, First thing I would do is gather the thinking heads together and try to figure out with one of Noel or Okafor your better off keeping. Okafor is probably the better prospect, but in terms of things like trade value, system fit, character and leadership it really isn't that clear cut who your better off keeping. But they won't ever really fit together and the longer you force it the lower their trade value. Then you move the one you decide to move for the best return of 2016 picks and young prospects you can find.
Figure out if Brett Brown actually has a place there. See if he has a vision for what kind of team he wants and if that vision is worth buying into.
Stay on the prowl for any young guys you might be high on that could be undervalued. Just for example, if the C's wanted Covington you would try to get Mickey if your keeping Jah or Hunter/Young if your keeping Noel. Make a commitment to the system you want to run though, and target undervalued young guys who fit that mold. Try to use one of your late 1sts or two on this type of move come around draft time. Never ever sign Dion Waiters.
Your gonna have another high pick, or maybe even two, this year. Stay flexible. Listen to any and all offers. If you don't get Simmons, and you don't trade back/out for established but young good players (Oladipo), draft a guy that fits what direction your team is going not just purely the BPA. If you keep Noel and Skal Lab is on the board, take him if you like him. Dunn from Providence if you move somewhere to his range should you keep Okafor. Or maybe trade Boston #2 for #5, #15 and Terry Rozier if you like him (or another guy from another team, Boston just has a lot of pieces to play with too). Be ready for any situation to seize on it if it gives you a broader base of young talent to start building an actual team with.
Most importantly is start developing some kind of united front of continuity and development. They have to start building toward some style of play with some competitiveness, fire and passion around that locker room. Get guys with a Crowder or Smart like personality to establish some leadership and accountability. Get at least one or two more veterans. Establish a core group of guys at different positions to develop and build a team around their strengths and weaknesses. It can't be a collection of d-leaguers and a few blue-chips, it's gotta be a couple core guys your most highly invested in and a bunch of other guys you have medium-low investment in. Have Saric come over NO MATTER WHAT. There's a lot of assets to play with there, but none of it really fits together. Identify a few you wanna keep and build around, and start turning the other ones into a cohesive roster that may be young and win 25-30 games but they'll be cohesive, competitive and be establishing an identity around a core group of young guys that you think will fit together and grow together, using your remaining pieces to finish the puzzle.
At least then you'll have re-established credibility in the league/with agents and other players, so guys might actually consider going there too.