If you are looking for ideas on how something like this might work in terms of Aspects, what a character might need, etc than consider watching the movie The Order with the late Heath Ledger in it. While the movie is about priests who are members of a particular Roman Catholic order who learn and counter diabolism and the occult, it sounds sort of like what the player is going for.
In terms of special abilities, I would definately recommend getting Thaumaturgy, either immediately or as an advancement. Aside from potentially allowing the player to bind, banish or exorcise entities, it can be used to 'bless' (enchant) items, Ward places, learn information, etc. Used properly and with some thought and foresight, the rituals available via Thaumaturgy can really allow a player to tilt situations to their advantage.