you get the whole game when you download it. Theres only a few DLC campaigns, but they can be purchased with in game currency...and honestly they dont make much of a difference except for story purposes...the game is mostly about PvP, not PVE.

The only thing you "have" to buy, is the event Campaigns which reward a Limited Rune (limited-edition, alternate art, sometimes with an emote) upon completion. A few of them are available for in-game currency, but most of the others are part of how they sustain themselves. Almost all Limited Runes, and all other Runes can be traded for, as well as Tokens and Tickets, although I'm not even sure Tickets are a thing...
But if you don't care about them, you literally never have to spend a penny, as it is.

Ok. Thank you for the comments :)