Int, Crit, Spellpower, basically in that order from what I've found. The haste will naturally come with the gear/talents, and you don't need to gem for it. Having a massive mana pool increases plea's effectiveness the most, you gain crit and spellpower from more intellect. 1k+ plea ticks are nuts when your all buffed up and rolling along in raid/groups.
21int+mana return meta, and int gems where possible.
Int int and more int. I have a couple spell power gems in my gear from before I saw the light and are mostly in pieces I will be replacing soon anyway. Your goal will be to reach a big enough mana pool that you can holy light spam non stop and last 5+ minutes.
My hol is a 1.42 second cast and I can chain it non stop for almost as long as I want. This is accomplished because of divine plea and replenishment's insane scaling based on your mana pool. I am doing nearly 70% overhealing anyway so I do not value spell power all that much. I have also glyphed flash of light because I never use it as a normal heal anyway.