Actually, you don't need to buy cards for casual games and the leagues that are currently played. Simply open the deck builder and start adding cards. There's a section at the bottom which will let you know if a deck isn't legal - if you've added 9 sites and it tells you that you haven't, simply add and remove a card and the deckbuilder will refresh. If you ever see me around, I'd be happy to play whichever format you prefer.