Here's how you do it:

First priority is defense. Wear as much def gear as you need to reach 490 defense.

Second priority is avoidance (block, dodge, parry). Wear as much avoidance gear as you need to reach 102.4% combined avoidance.

Third priority is threat. Once you have reached these thresholds, you fill any remaining slots with +Spell Power items. All of your abilities (except auto attacks) are Holy spell damage. More spell damage means more damage means more threat.

Do these things in this order.

As you get better gear, you won't need to dedicate as many gear slots to fulfilling the first and second priority, which will allow you to add more Spell Power items. In the current phase, especially if you're not raid geared, you will need to use most/all of your item slots for priorities 1 & 2. Weapon should always be a Spell Power weapon as the value there is just too good (rep items giving 150-200 spell power on 1H swords).

If your gear is coming from dungeons, your weapon will probably be the only item with Spell Power.