Honestly, I'd hold off on any major purchases until after rotation (late March or early April). We will get a new Core set then, and three entire expansions will disappear. Every archetype that exists right now could easily become a meme. HOWEVER, you definitely want to buy the mini-set when it appears in the next few weeks. Those are always the best bang for your Hearthstone buck.
Until then, you can earn gold by completing your quests in Duels and Arena (and to a lesser extent, Mercenaries and Battlegrounds). Arena will cost you if you aren't a strong player, so Casual Duels is probably your best bet for completing quests. If you "git good," you can start playing Ranked Duels and earn gold on top of the quest completion.
Also, maybe get the Rewards Pass so you can get the accelerated XP -- but this might not be worthwhile if you don't play enough. Get it if you like the portraits, I guess?
If you absolutely must buy cards before rotation, stick to Barrens, Stormwind and Alterac ONLY. Unless you're a big Wild player, you won't get much use out of the older stuff before it's gone.
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