The value of stats vary depending on your current stats. Use tools such as wowanalyzer to evaluate your stat priority.
Mastery > Crit > Versatility > Haste
In general, no one stat is so much stronger than the others that you would sacrifice total stats to gain more of a specific one. So for the most part, prioritize item level (ignoring gems and leech, of course). If two items are equal item level choose the one with more mastery and crit.
Mastery (Mastery: Echo of Light): Your direct heals leave a short HoT on the recipient. Mastery is your strongest HPS stat.
Critical Strike: Increases your healing, but has no special interaction with any of your spells. Nevertheless it is a very strong stat for Holy Priest.
Versatility: Increases your damage and healing while reducing your damage taken.
Haste: Increases your cast speed and reduces the cooldown of Prayer of Mending, but overall is your worst stat.
I recommend choosing your race based on the tools they give you, not the minor differences in output. You gain more value from an extra tool to help you deal with mechanics, movement, etc, than from a 0.0001% difference in throughput.
For Alliance, the best race for M+ is Night Elf because Shadowmeld is extremely good in dungeons. For raiding, I recommend Void Elf. Holy Priest has extremely weak mobility, and Spatial Rift helps fill that gap. No other racial will do more to improve your survivability and ability to perform mechanics than adding a powerful mobility tool to your kit.
For Horde, the best race Holy Priest is Goblin. No other racial will do more to improve your survivability and ability to perform mechanics than adding Rocket Jump to your kit.
Mastery Gem: Masterful Sea Currant
Food: Abyssal-Fried Rissole
Rune: Battle-Scarred Augment Rune. The new Rajani faction in Vale of Eternal Blossoms will sell a reusable Augment Rune for 50,000 gold once you are Exalted with them.
Potion: Prepot with Potion of Unbridled Fury in almost all cases. If an encounter begins with immediate damage requiring healing, you may prepot Superior Battle Potion of Intellect instead. If you require mana during an encounter, use a Potion of Replenishment. Otherwise, use a Superior Battle Potion of Intellect to boost your healing, a Potion of Unbridled Fury to boost your damage, or a Skystep or Lightfoot Potion if needed for mechanics or to avoid death.
Healing Potion: Use Abyssal Healing Potion to save your life once per fight.