The biggest reason I ask is because I've played said character (the champion of god, with soulfire in lieu of other powers). All you need is an initial firm idea of how your powers will work and what your sponsor's agenda entails for you. I was playing a character with a firm bent towards justice and righteous wrath, so for me the sponsor's agenda had to do with righting wrongs, protecting innocents (or thereabouts), and destroying those who would harm others. I determined that my soulfire would be about protective light, white flames, and (because the character had the warrior and the nurturer fighting within her) calming soothing healing power. Then I simply did what fit within that template. If the GM called shenanigans on anything I was doing (which happened rarely), I gave him my reasoning for it and he made a call. It wasn't that hard.
If you're worried about the cost, Soulfire costs exactly as much as evocation, holy magic and holy touch combined and does everything those powers do (with the limitation that you get more powerful effects for the cost of mental stress).
If you're worried about the integrity of the template I say the templates are only useful when they're helping you. When they get in the way of making a good character then it's time to chuck them.