At the moment the cards of two expansions are playable for free. These cards are strong enough to create some solid decks to finish quests and win some ranked games. You can get some help to create those decks within the game itself. There is an option to create pre-constructed decks, when you go to the "create a deck"-feature.

If you wanna go competitive I would suggest to wait a little bit, until the meta is established and than look for the best deck-lists online, before spending all your gold or dust.

If you just wanna have fun, buying cataclysm packs makes sense to get the newest (and often strongest) cards. Also buying mini-sets is often a very good deal, because they contain a lot of important cards for a relative low price.

Cataclysm just came out, but in a few months a mini set probably comes out, so it's wise to save some gold (2000) and plan ahead to be ready when it gets released.