As far as heroics go, any tank can heal, but the Prot Paladin is much easier to run.

You will want your heroic team to include the toons you want to gear.
Badges can get you PvE or PvP gear, and if you want to gear a DK then the DK should be in the team.
If you're happy to build the best PvE team, and go with that as is, then the ranged team is easier.

Priest is probably the top pick for a healer, especially with a shaman already on the team.
I'd probably take a shammy healer over a priest, if the team did not already have at least one shaman.
With two elemental shaman, the priest will be a far easier option to heal a team with.

So you have Prot Paladin, Disc (or Holy) Priest to heal, 2x Elemental Shaman.
I'd think the perfect last addition would be either another Elemental Shaman or a Warlock.
Boomkin works too, but then you need to manage eclipse procs for good dps.
The Shaman is easier, you already have 2x.
The warlock gets you the 13% spell power boost, and can be /Click dps like the Shaman.
The druid also gets the 13%, but you need to manage Eclipse procs to match Shaman/Warlock dps.
The mage isn't a bad choice, I'd go with arcane for now; in 3.3, frost gets a persistent pet which should help the spec a lot.
At the moment arcane can just spam Arcane Blast, but a rotation of 4x Arcane Blast and 1x Arcane Missiles is better.
The frost spec is Frostbolt spam, with the water elemental on every cooldown.