I think Perkins and Rondo could definitely co-exist with each other, like they do now. I think the key is to find an athletic PF for Rondo to run the break with. Perkins is a necessity as he gives us freedoms on defense such as not double-teaming elite post players. Fast breaking teams with fast breaking centers do not go far in the playoffs.
In terms of players I would like to acquire, Melo would definitely be 1st. He is the best pure scorer in the league, IMO, and is a top 5 player in clutch situations. He also would give us a post-game which would allow us to grab an athletic defensive PF (generally these players are not back to the basket scorers). This is more realistic than Durant because of the Melo's growing frustration with the Denver's playoff woes.
Another player I would like to see on the Celtics is Amir Johnson. I think he would be a perfect backup for Perk, giving us a different look a C with his athleticism.