I'm using Boston Unveiled for setting, plot hooks, and NPCs, but I've put my own twist on some things of course. Glad to hear you're thinking of STing Mage! I recommend Tome of the Mysteries as the first supplemental book to pick up. It adds rules for alchemy, explains magic in greater detail, and really functions as a second core book. Sigil and Sanctum is a great one for understanding how mages work together as cabals, and STs can get a lot of mileage out of that one. By far the most inspiring book for a storyteller is a newer one, Keys to the Supernal Tarot. It's just full of plot hooks--one for each card in a standard tarot deck, in fact, so you can use an in-character tarot reading to generate story ideas.
Other books are profoundly valuable, too, but more dependent on the type of story you're telling and the inclinations of your players. There's a book for each order, and each of those makes for a great afternoon's reading if you enjoy the setting; if your players involve themselves in mage politics, you'll find them very valuable. Intruders: Encounters With the Abyss is about including Abyssal entities into your game, very dark and horror-oriented with about five pages dedicated to each entity. You'll want Secrets of the Ruined Temple if your group is looking for an "Indiana Jones" feel to the game. And the new Seers of the Throne book casts the one of the major enemies mages face in the game in a very intriguing and thoughtful way.
I would not recommend either of the Legacy books for an ST, though. There is some story potential in them, but they're mostly for players looking for options. If you've played D&D 3.5, I'd compare them to a book entirely of Prestige Classes: players salivate over them, but you'll probably only use one or two of the legacies in there. There's not a lot of bang for your buck.