You don't /have/ to buy your decks in TCGO. You can play with the Theme Decks you get from the game and unlock packs, buy decks or more cards with Coins, trade boosters you win in Online tournaments, etc. And eventually, if you buy any TCG product, just upload the code. It's definitely slower than someone that uploads codes constantly and has a lot of cards, but it's a start. And TCGOne is mainly just for play-testing or playing formats literally older than you. So I'd suggest that you try out the official client and then, once you have enough practice, use TCGOne.