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20k-30k damage is fine, especially since I'm wearing bad gear. First, there will always be a healer in instances. Second, there are also healers in battlegrounds, but you only need a healing build if you're leveling. Make two specs and switch between them when needed, or stock up on health potions.
Added 2 minutes later
By the way, it's easy to live without a healer. I enchanted my Death Knight's gear to reduce damage from monsters, so they barely do any damage to me.