I think your assumption of all 3 of Brown, Tatum and Smart is way too rich a deal and Ainge won't give up all three.
Either Smart or Brown will be on the team next year if the Celtics trade for Davis.
A point guard not named Rozier(Darren Collison, Patrick Beverly, Ricky Rubio, Jeff Teague, Derrick Rose are all possible),Brown, a healthy Hayward, Davis and Horford can be enough to contend.
I very much hope that Ainge won't trade all three of those guys to get AD.
As for whether the team you mentioned would be enough to contend, I disagree.
I just don't believe a team can be a serious contender for the title with a mediocre starting backcourt.
On top of having an average-at-best lead ballhandler, that team would need either Hayward or Brown to turn into a legitimate second scoring option from the perimeter. The Utah version of Hayward probably counts. Is he going to get back to that level next season?