I am a casual player and provide the cards for my opponents (though several of them have expressed an interest in buying their own cards). I started with the Core Set and bought Assault on Uluthan next. Then I bought some of the Corruption Cycle packs. My next purchases after those were March of the Damned and the Legends expansions. I completed the Corruption Cycle and now I buy two Battle Packs at a time when my funds allow. I buy one pack from an older set and a pack from the Eternal War cycle. That way I have the new shinies (like those talked about on Mallumo and Happy DD's excellent podcast, Winvasion) and am also building my collection with older cards.
A word about my deck construction: it is quite unorthodox. In an effort to play with every card I own and not intimidate new players with established decks, I randomly build decks. One for each of the capital board factions.
I shuffle all of the units of a faction and deal out 25 of them face down. Then I shuffle the support cards of that faction and deal out 5 cards, do the same for tactics cards. Then shuffle the quest cards and deal three, Legends and deal three, add the epic spell for each faction, and then deal out 8 neutral cards (divided by order and destruction, of course with true neutral cards randomly dispersed between order and destruction) to round out the deck.
This method seems to generate surprisingly balanced decks as many of the games we have played have been a blast and not runaway victories. Every so often, I 'reset' the decks. Usually after receiving a new Battle Pack or Deluxe expansion.
Hope that helps.