I'd suggest saving your stars for the upcoming Sealed Deck mode. Nothing is set in stone but when they tested it last year, it cost 5000 stars to join.

The Welcome cards are also a nice buy especially for the meta ones. They are not yet minted as NFTs right now, but they will be eventually. Which means they can be sold in the future.

For the core cards, I feel like you should only buy them if you don't own them and need them for a deck.