Which mode do you intend on playing most? If it's standard, I'd highly recommend paying close attention to the top decks there and craft the cheapest ones you can afford and do some resource grinding. It takes some time but should be doable.
If you wanna focus on any one set I'd highly recommend United in Stormwind. Easily best set (In terms of power level) of the year and its filled with the most powerful and broken cards and a lot of current decks are centered around those recent Legendary Quests.
I'm in wild and there, Pirates and Tavish hunter variants are HUGE. Both decks are getting hit with the latest nerf patch but don't let the patch fool you, the meta won't change much. Hunters will merely forgo Baku if need be and it will still be a strong deck. Both tavish and pirates are relatively strong and cheaper decks to craft in wild.