I'm not healer main but I think caster or buster doesn't really matter for a healer, there's a small bonus with crit rate/damage? when enchanted, but overall it's not something that will influence your healing at all. As for the AP, I'm pretty sure the knight tree is the best. casting speed should be easily reachable, even for a dark elf. I think M.atack does have influence on healing, but you need a lot to see the difference, and AP tree is not gonna give you that difference. As for skill power, I was told it doesn't work on healings. in any case, I think your main healing power will be balance heal and progressive heal rather than the regular heals, except probably for panic heal. Focusing on defensive stats is your priority at any time, and the wizard tree doesn't provide you enough of that.
Hope somone more experienced can confirm what I'm saying.