Starcraft 2 Legacy of the Void - unlock heroes for co-op
It is okay to buy Starcraft 2 keys from resellers (and often cheaper than from Blizzard directly).
After they send you the key, you activate in on your Blizzard account and the account gains that game license - the same end result you get when you just purchase from Blizzard.
As been said, yes, you only need Starcraft 2: Legacy of the Void to play Swann, Zagara, Vorazun, and Karax.