I'd be surprised if it makes any sense for the Celtics to sign Kaman for the next season.
I'd also prefer to keep Garnett at the 5 for as many minutes as possible. However Bass is a pure 4. Bringing in another pure 4 would imply relying on Stiemsma to have an important role in the big man rotation. A 3 big man rotation, with Bass+new PF seeing time together, wouldn't work.
The ideal solution would be a 4/5 like Varejao. If that's not a possibility, go to the best big man available, regardless of position.