Healing is the most demanding role for keyboard players, because of how much party/alliance targeting they have to do. You'll need to come up with an effective way to target party members and mobs when you heal. Personally, I prefer mouseover heals with the spells bound to chords that include mouse buttons. Other options are using your right hand to tenkey target or bind your targeting keys to chords to put them all in reach of one hand (e.g. F1,F2 for tank1, tank2; aF1-aF5 for DDs 1-5; shF1,shF2 for self and healer buddy).
DDs and tanks can easily be controlled with keyboard only, provided you have redundant movement keys, so that you can move with either hand or both.