So how does deck customization, extra decks, and such work? There are like 400 different purchase options and like 50 different decks so I have no idea what I'm getting if I just buy the regular 10 dollar DotP 2012 game.
Technically, you can play the game by just buying the bare, raw version.
Then there's the expansion, which gives you three more decks and an additional additional Archenemy mode. Then there's the two deck packs, which give you 2 more decks each. That's pretty much everything the game has to offer, and it's pretty fun, I think, especially if you have some friends to play archenemy with. You can also just play random matches with other people online, and they won't cheat like Koth.
Everything else is literally just 'unlock all cards for a certain deck' or 'turn all cards of a certain deck into shiny foil version if you think you have too much money'.
You can unlock all cards just through hard work, but the foils you need to buy if you're into having everything in your hand glimmer and gleam at you nonstop.
Deck customization is very easy. You unlock cards as you win matches, any matches, and they're added to the pool of cards you can use. You may take out or leave in any cards you feel like from the customization menu, but you can't control land ratio which is a pain.
Also, you unlock more decks from playing the campaign, or just straight up buying the deck packs they're unlocked automatically.