I played a lot of Ars Magica at university and a few years after though I've only ever read the core book, so this recommendation is a bit less familiar. Easy one is for DriveThruRPG bundle with all Atlas Games Ars Magica 4th edition products for $80.

Ars Magica 5E has 40 books out so it's quite well established. I like how well organized the supplements are. Not that many good adventure books though so I'm not sure what supplements to buy instead.

Ars Magica 5E ($20 PDF)

Houses of Hermes: True Lineages ($15 PDF)
Houses of Hermes: Mystery Cults ($15 PDF)
Houses of Hermes: Societates ($15 PDF)

These three books cover the twelve orders of magi. I'm hesitant with splat books and the core rules must describe them adequately but Ars Magica is to a certain extent is about each magi being their own island so more details are good.

The Broken Covenant of Calebais ($10 PDF) - a 96-page scenario
Tales of Mythic Europe ($15 PDF) - 144-page supplement with 9 one-session scenarios.

The other adventure supplements are interesting but don't seems simple to use: high level PCs, multi-year storylines, world-ending plots, locations and myths. More for experienced GMs.

Hooks ($10 PDF) - the above being said with $10 left this 96-page book has eight one-session scenarios that leave room for more exploration if PCs want to pursue further. The description says each hook leads to an area further explore in another supplement but I guess a GM can make stuff up.