So, I just started playing like a week ago so I don't know too much about the issues with the core cards. Honestly, their are a bunch that I run in my decks, but that's basically because I'm WAY behind on cards from any of the expansions. I would assume cores could be overhauled every other year, though that would be a lot of work on Blizz's part.
I can't put my finger on it, but I think I heard somebody say the next expansion rotates through in 4 months or so. :D
The expansions from 2017 will rotate to wild. Journey to Un'Goro, Knights of the Frozen Throne, and Kobolds & Catacombs. As a new player you likely should not be buying packs from these sets or crafting cards from them. If you do in April 2019 you wont be able to use them in standard format. Since you dont have enough cards to play wild they will be useless to you.
Please take a look at the post in my signature, it may help avoid a few headaches.