Are you going to be playing Wild or Standard? Some constructed players also like Duels, so maybe give that a try.
Finish your dailies, weeklies and grind the Rewards Track. There are a few free cards on it and the rewards are significantly improved over those you'd have got during the K&C era.
Consider purchasing the last miniset and the one likely to be released next month. They are hard to beat for 2k gold each. Each miniset to date has consisted of 66 cards, including 4 legendaries and 2 epics.
Personally, I'd probably wait to pick up the Mega Bundle for the next expansion ($80). You could purchase the Tavern Pass ($20), but it might be a bit late. Again maybe worth waiting until the next one in April.
Oh, and get a heroic Duels run to two losses before the reset later today. That'll net you a Tavern Ticket and a pack at the very least. More if you do well!
Regis Killbin is good for Duels. Kibler is one YouTuber, around during K&C, who is still playing Standard. It shouldn't be too hard to find someone doing Battlegrounds. It's the most popular game mode now.