Once you are ready to start building a collection, buy CoW packs first. They've got the newest/greatest cards right now - definitely a value play as well, based on prices of the individual cards. Plus you get a bunch of older cards that might land you some solid cards from past expansions.
TEC packs are also a great buy, because you get 3 FooF and 3 RoA cards with each TEC pack.
Keep an eye out on the Trading board as well. From time to time, players want to sell their collections. These get you the best "bang for your buck" in terms of buying cards. But you can never predict when players will want to sell. And some collections are valuable enough to warrant high prices. Still, if you can pick one up, this is the best way to jump-start a collection. You'll spend so much less in the long run if you can grab one of these.
You can also post what you are looking for on the Trading boards and ask for offers to sell you cards. FooF/RoA cards flooded the market with the TEC expansion, so you can probably pick up complete sets of these on the cheap. And these cards are recent enough that many can be used in competitive decks. I know that I personally have a massive pile of FooF/RoA cards that I need to sort or sell. Just make a thread saying "I want X, Y, and Z. Please PM me your offers to sell me any/all of these" or something like that.
I would avoid buying packs of anything Priests or older for now. Most of the cards are not playable, and you'll be better served buying individual cards from Three Lions Gaming from those expansions.
And I agree with kariusvega - download Lackey and start playing online!
