What ur going to have to do is have a shift, alt, or some other button with 1, 2, 3, 4, for ur heals.. (which are more frequent) and make ur iceblock o crap ones like F5 and stuff.. Or if u get/have a keyboard with extra keybinds i would utilize that..

For when u have to heal AND dps---
have a heal+dps button.. (to be reaaally simple).. the mage gets fireball, DK gets hit something, and the holy paladin gets a heal.. all on the same button..
OR
create a macro that has target=help /cast healspell ... target= nohelp /cast dps...(someone else will have to help u with this macro im sorrry i dont know it.