I can't really help you with Lipo stuff. I don't run them and don't know a whole lot about them.
I do know that both of the chargers you mentioned are good chargers and will do pretty much everything you need.. if you want to spend that much dough.
It's called cell balancing, basically I think it just makes sure that all of the cells in the pack are very very close in voltage. It is very bad to over discharge Lipo cells, so if one cell happens to have less voltage than the rest in the pack it will probably get discharged more than the others... not good if you run the pack too long.
Don't know about the heat sink. I think the mamba cans are smaller in diameter than 300 motors but I could be wrong. If you are getting a Factory Team RC18T then you will already have a heat sink motor mount (comes in the kit). It is not a wraparound type heat sink but it should help draw heat away quicker than not having one.