I would definately use a shaman healer. I think your main problem will be maintaining mana to get through a whole boss fight. Using earth shield and riptide you will have alot of time in the 5 sec rule meaning you will regen your mana. Hots from a tree druid need to be kept rolling and with the different times they last you' be casting often to keep them up. Also you would be wasting the effectiveness of wild growth when you only have 3. Chain heal would be more efficient.

I would use a moonkin for the dps. Gives you an innervate and since you will only have 1 or 2 dps the eclipse procs would be easy to manage. With a sham+druid you can always switch the healing/dps/tank roles around and find out which you like better too. A hunter won't ever be able to switch to heals or tanking. Also a pally/sham/druid use 3 of the 4 armor classes so it will be easier and more fun to gear them.