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.