Oof, @Timothy_Ferguson may have some suggestions for you as a fellow Aussie.
Minimum, I would say is a core book (obviously) and Covenants. You can use e-books for SG knowledge on anything else.
You are certainly not the first, and you probably get some good advice reading some other threads:
New to Ars Magica - Games Discussion / Ars Magica - Atlas Games RPG Forum (atlas-games.com)
Coming from a D&D group, they will be used to seeing themselves as The Good Guys (stereotypically, IMO) and you are not necessarily such in Ars Magica.
My general advice is:
- House Rules are the best, don't be afraid to make them.
- Do not force everyone to make a grog, and a companion, and a mage, before you play. Let each player pick one type to start with.
- If you house rule, - virtues and flaws do not have to balance. I wouldn't allow more than 10p of Virtues as a new GM, but make them pick a couple flaws they find interesting.
- Make sure you actually sell the game to your players and have them understand what they're getting into.
- Combat is not the solution in this game, and the rules are (1) a bit wonky and (2) very lethal.
- Remember that Healing is expensive and much harder to do in this game.
- Really talk over what kind of game and situation you and your players WANT to explore.
- Let the players rewrite their PC's if they later realize they designed them wrong.
- Related to house rules, as D&D players they will quickly realize they have more Characteristics (Abilities in D&D words) but less points. I often let my PC's have a flat 7 to 12 points to put in anything they want)
- If all the players want to be magi - fine! Let them start as apprentices if they desire, to get a sense of how the game works first before having to full stat out their PCs. You could just use half the standard XP and assume they're 7-8 years into their apprenticeship and teenagers.