If you plan on playing Standard, then craft a cheap Hunter deck that will last a few months, especially if you’re ftp. If you’re paying for packs, then I’d say buy 10 of every expansion. Like others said they’ll be useful in Tavern Brawls and Wild if you choose to venture.
Zul’jin is worth it in any deck that plays 10 spells or more. Especially if you’re running spell stone and stall secrets. First craft this expansion and he’s incredible(when I actually play hunter...).