A friend of mine who was around rank 3-4 says you'd be fine if you drop like $150 on packs, for dust to craft, mostly.

If you don't want to spend money on the game (I didn't) then do the quests for gold, and make sure you get a free pack from every new Tavern Brawl (Tavern Brawls are fun, too, since you get random cards most of the time)