Depends on what you want to play.
Songhai really just needs spirit, so buy core.
Lyonar NEEDS Oath and Scintilla, so buy RotB.
Vanar needs both RotB and AB.
Vet is hot garbage w/ out Blood of Air, so you need to buy AB.
Magmar needs spirit and Lavaslasher, so buy core and AB.
Abyssian needs a LOT of spirit and Punish so you'll want to buy core and RotB.