Using advanced metrics (Wins Produced) courtesy of 
the NBAGeek, we can make a decent prediction, I think. 
We have KG, Rondo, and Pierce back. Last season, the 3 of them were responsible for the equivalent of ~34 wins (adjusted for an 82-game season). Honestly, I expect KG and Pierce to be mildly less efficient this year than last year due to aging, but not by a huge margin, and I think Rondo will improve enough to more than make up for that loss. For now, let's just set this 3-some at 34-5 Wins Produced.
Ray Allen's Wins Produced for this past season was 6. Jason Terry will give us 4-5, based on his performance the past 2 seasons. So that's a slight drop-off ... however, gaining Courtney Lee gives us another 4-5. So that's 8-10 from the two off-season pick-ups, bringing the team up to around 42-45 wins.
Bass, Bradley, and Wilcox gave us around 6 wins last season, adjusted for an 82-game season that would be about 8. So that gives us 50-53 wins. 
That isn't too bad on its own. I think Sullinger will be just as effective as Bass this season, so I expect him to give us another 3-4 wins on his own, bumping us up to 53-57 wins. I think that's a pretty safe guess.