Triple Boxing

[nodoze]

It sounds like you would main a Mage for at least BattleGrounds. Based on your goals, since it seems you are more PVE focused, I think either party should work. If you plan to mainly use the Priest/Shaman slot as a Healer then Priests are pretty compelling as, though somewhat squishy, they are probably the best all around healers in the game (though personally I am more of a fan of Shaman so a lot of it comes down to what you want/like).

I am not sure I would go all Orc or all Undead for either party composition (especially if mainly PVE focused) unless there was a compelling reason to. Generally speaking it isn't that big of a deal (and only a 1 time up front effort) to get characters of different races together for leveling. I think the following are the recommended races for the classes you are considering:

PVE Warrior: Orc or Troll (I like Orc much better for Warrior);
PVE Mage: Troll;
PVP Mage: Undead;

PVE Shaman: Troll (for both Resto & Elemental);
PVE Priest: Troll;