All of the above are good suggestions. The core set is enough to get you going for sure, and that in itself is already a ton of fun for up to 4 players. And that leaves plenty room for card drafting and such.

But I suspect you'll sooner or later want more copies of some go-to spells (far from necessary though, and any mage can still put up a good fight with just the core set). If your group tends to get competitive and fight over specific cards, then a second core set or the tomes would be a nice thing to have.

I provide the games for my group as well, so after picking up the first core set and trying out a 2 vs 2, I quickly bought a second core to double up on the cards and for the extra board, markers and tokens. After that, I got both spell tomes and FM vs WL.

That allowed me to have all 6 mage spellbooks ready to rumble, with a ton of cards left over for deeper customization. The only thing I'm missing are a few more Hands of Bim-Shala to give each of the mages. No biggie though. Otherwise, this setup has pretty much all bases covered.

Penny sleeves do the trick. Sleeving everything is expensive, and there are a ton of cards.