Always try to simulate your character using raidbots or the simulationcraft program to get personalized results.
Single target / Multi target:
Intellect: Increases the magnitude of your spells.
Critical Strike: Chance for spells to do double damage. Shadowy Apparitions spawns an extra ghost when you crit with Mind Blast, Void Bolt or Devouring Plague and Hungering Void extends by an additional second when Void Bolt crits.
Haste: Increases spell casting and dot tick speed, reduces the cooldown of some spells and lowers the global cooldown.
Mastery: Increases your damage on the target for each Shadow Word: Pain, Vampiric Touch and Devouring Plague effect that you have active on the target. During Voidform all targets receive the maximum effect.
Versatility: Increases damage and healing done and reduces damage taken.
The best way to calculate stat priorities for your character is to "sim" your characterFind out how to sim
The differences between the races are very small so you should play whatever race you like the most.
Goblin: While the 1% haste you gain has value, the biggest reason you should pick a Goblin is due to Rocket Jump. The added mobility is a lifesaver in many situations.
Mechagnome/Void Elf: Mechagnome sims the highest, but for the same reason Goblin is so good for Horde, the added mobility from Void Elf will give you more value on average.
Bracers: Eternal Intellect
Note: The better gear you have, the stronger Celestial Guidance becomes, so don't be surprised if it starts to outsim Sinful Revelation even on ST.
Weapon: Shadowcore Oil
Chest: Heavy Desolate Armor Kit
Flask: Spectral Flask of Power
Damage potion: Potion of Spectral Intellect