So, I'm thinking about building a Maester deck at some point in the future. I know I'll need to pick up the Secrets of Oldtown packs, but what older expansion packs should I pick up?

There are currently 15 Maesters in the game. Martell has the majority of them. The Core set comes with a few and the other sets which provide them are:

Baratheon - Maester Pylos in Kings of the Storm

Greyjoy - Maester Wendamyr in Kings of the Sea

Lannister - Qyburn in Secrets and Spies. Grand Maester Pycelle in The Raven's Song.

Martell - Maester of Lemonwood in Beyond the Wall, Maester of the Sun in A Song of Summer, Maester of War in The War of the 5 Kings, Refugee of the Citadel in Refugees of War, and Starfall Healer in Return of the Others.

Stark - Maester Vyman in The Winds of Winter

Targaryen - Maester Aemon in A Change of Seasons