Welcome to the Splinterlands :)

When I started 6 Years ago, I tried to own at least one copy of every card. This gave me the most versatile deck and opportunities. With the grown amount of rule sets this is more important then ever.
After that I look for the next breakpoints of the cards. Where do I get an additional attack point or a new ability. But also think that you need summoners that allow you to use this level of cards.
Since there is modern and wild format, I would focus on modern cards. Chaos Legion will rotate out in April for the next set. So cards from rebellion might be more interesting in the long there.
Since there was an overprint of the Chaos Legion set these cards are cheap to buy and to rent.

I hope you enjoy playing!