Lexrageorge has laid out the numbers above. I would add that the projected non-taxpayer MLE is $8.8M and the biannual is 3.3M. The player selected with #27 will cost around $1.6M (120 percent of $1.367M). Today the Celtics have $107M committed, which includes Smart's $6M QO.
I'm guessing Smart will be gone one way or another, via unmatched offer sheet or sign-and trade, and that Baynes takes the $5M. If Baynes doesn't, Kyle O'Quinn (player option) might be a cheap replacement. They will have Yabusele for one more year unless they can trade him. Or maybe he comes around as a player.
I'd like to see them add a guy like Dewayne Dedmon with or Derrick Favors with the MLE, although either one may be too expensive. I'd want to add a shooter like Joe Harris with the BE.
As for trades, I'd like Ainge convert the Sacramento and Memphis picks into a top pick in this draft, because this year's draft is so strong. It may not be possible. Will Ainge look to trade Rozier now, when his value is high? A young big man has to be a priority for this roster going forward.
Another interesting question is what happens in 2019-20, when the cap is projected to be $108M with the tax line at $130. Will Ainge pay Kyrie 30% of the cap or look to trade him? Will Horford exercise his player option? If Rozier is still around, what will they do about him?