I wonder if the UV-18S (Twin Otter)(see more at http://www.fas.org/man/dod-101/sys/ac/uv-18.htm) might be a very good fit as well. The Army Guard uses them here in Alaska to good effect. Seems like the sandbox would be a good fit for high speed medevac, fast ammo resupply and so on.
I should think that Otters and Beavers would both work well.