If I was going to offer you one piece of advice (and I am, apparently), it would be to start - and play - all your characters together from day one.

Outside of instances, your tank and healer will just be leeches most of the time, but the value of doing that lies in learning how to play them all together right from the beginning, when things are a bit more forgiving.

The healing question is entirely down to personal preference.

I prefer a holy priest as my primary healer when I'm boxing, because he has the "oh God, it's all going wrong what button shall I press?!" spell that is Circle of Healing.

As a secondary benefit, his holy nova synergises really well with Hellfire / Arcane Explosion / Consecrate for PBAoE nuking.

Any healer will work though.