I think that basing your concept of Soulfire purely on what you saw Dresden do is a mistake. I believe that in most cases, Harry specifically describes using soulfire to strengthen a spell cast using a different element. In the case of the giant fist, I think it was Spirit (force), for example.
The book is (as always) pretty vague on what can be done with Soulfire, but it seems reasonable to start with that: Soulfire is in many ways like standard Fire, at least in terms of evocation. So that gives you a wide variety of evocations based on fire, though of course your fire will (a) count as Fire for purposes of Catches, (b) count as Holy for purposes of Catches, and (c) partially satisfy any other Toughness Catches (reducing Toughness by one level). Pretty potent. If the character also has a normal evocation/channeling power, then these benefits transfer to spells cast in the other element. Also very potent.
In addition, Soulfire as "the rebar of creation" seems to have a strengthening aspect to it. I'd consider allowing spells that make people or things stronger. This might include raw strength, resistance to damage, defenses against attacks, healing/curative magics, etc.
In terms of limitations, I'd say that the GM should make good use of debt compels (or even compels against related aspects) to ensure that Soulfire is not being used in ways it wasn't intended to be used. Attempts to use Soulfire to satisfy greed or other 'sinful' motivations should either fail outright or come back to haunt the caster in some way. Attempts to use Soulfire against good people should probably result in the magic simply not working, or working in a reduced capacity. Maybe the cleanest way to do this would be to charge debt for any use of Soulfire when not used for sufficiently Righteous causes, and in some cases to use that debt to prevent certain uses of the magic.
Note that much of the above is speculative.
On a side note, I don't think that "Living Dead" works well in the context of a celestial servant. The Living Dead power represents a total unnatural, abherational return to "life". People see you and reject your place in the natural order. Your body does not function normally any more. None of this seems compatible with someone brought back to life by the Creator or his servants. Instead, I'd rely on an Aspect to bring your return to life into focus in the game, and possibly take a Recovery power to speed healing (instead of letting the pieces just fall off). Note that Inhuman Recovery would also let you get full benefit from your Catch, too. As to your question about its value, by the way, I would say that the proper value is +2 for "unholy stuff" (which would include unholy relics and attacks by unholy beings). Possibly even +3, as players will often find "unholy stuff" to be in greater supply than "holy stuff". That said, your Catch is always reduced to one point less than the value of the powers it modifies, assuming it would otherwise be greater.
