The Priestess is a cool mage, though you will find that you tend to devote more spell points for creatures and incantations than many other mages. If you end up playing Priestess, Battle for Kumanjaro is an essential expansion after the base set because of the Guardian Angels, which are still the best defensive units in the game (you will find that it usually takes four times their mana cost and their spell point cost in attack spells and conjuration spells to bring them down because of the combination of thei healing ability, dodge chance, and Aegis). After BfK, I would suggest getting Academy Priestess, Paladin v Siren, and then the Lost Grimoire 1 in that order (you will also be able to make a decent two more good spellbooks with that combination of cards), though you can take your time getting those, as I had a great Priestess spellbook years before they came out.