I think that there's a lot of potential for the character concept; I think you'll have fun with it. I think, though, that the Catholic Church as an institution would tend to take a dim view of a "Sorceror Priest", however. That said, I think there's a lot of story potential in being a member of a secretive order that did not go in with the "Suffer not a witch to live" thang, believing that such power could be a tool for good every bit as much as a tool for evil, when handled prayerfully. You could even write up the order as secretly under the direct authority of the Pope. Which means that you character, your order, and the Pope are ok with it, but you need to hide it from the Church as a whole ... or else.
I will start this with a question. The character is going to be an ordained Roman Catholic Priest or perhaps seminary student, correct?
If that's the case, I would suggest reading the short story The Warrior.
In it, Father Forthill reveals that he is a member of a secretive sect within the Catholic Church, the Ordo Malleus. The order has been in existence since the founding of the Church were it was tasked with performing exorcisms on the possessed. It then expanded its area of responsibilities to include combating other supernatural threats like vampires, evil fae, etc. It is suggested that the Ordo Malleus was actually behind the Inquisition, but that some members lost sight of the objective which is why so many people ended up dying. Since the Inquisition, the Ordo Malleus has apparently operated very quietly to support the Knights of the Cross and combat the enemies of mankind, and there is the suggestion that the Pope is aware of the existence of the Ordo Malleus and has some form of oversight in its activities. Some members of the Ordo Malleus also have contact with the Venatori Umbrorum.
For some other character ideas, the movie the Order might be worth watching.
In terms of modeling the character for the DFRPG, this question comes immediately to mind. Is the character mainly a Wizard whose jobs is that of a priest, or is the character mostly a Priest who can cast spells? That is important to know since Wizards have one set of powers requirements, while True Believers or Champions of God have another set.
If the current character emphasis is going to be a Wizard whose job is a priest, then having a greater emphasis on Evocation and Thaumaturgy would make sense (Thaumaturgy more so than Evocation in my opinion...) and that the flavor of the character's spells should reflect the character's faith-based approach to magic. In one of the books, Harry commented on how some members of the White Council are devout followers of one religion or another and that the magic of those members reflects their faith. Incidentally, it was also mentioned that the headquarters of the White Council at one point was located in the Vatican... Summer Knight comes to mind as the novel, but I could be mistaken.
Now if the character's emphasis is on that of a Priest, taking the True Believer template powers of Bless This House and Guide My Hand and adding one or more Spellcasting powers like Ritual: Diabolism, Thaumaturgy, Channeling: Spirit, etc. could make sense, and again the spells would have a flavor reflecting the character's faith.
Something which could be an important consideration as well, if the character is going to be a full Wizard, is whether or not the character is aware of, and/or a member of the White Council?