The ability buffs and poison saves are not ridiculous, and are comparable to Dragon Disciple. At higher levels, the buffs are less meaningful anyway, since many classes will have the higher ability boosting items anyway.
The three elemental resistances/immunities is quite powerful, arguably the best feature of the class. This is a potential unbalancer worth considering. However, Fire was excluded, which is probably one of the more common (and most damaging) energy forms.
The flight ability at 10th level is not a problem.
Of course, the spells come directly from the template, and are generally not bad. Holy Aura 3/day seems a bit much for an 8th level spell, but that's what the template says. I suppose it's not too hideous. And I am still concerned about Resurrection. Resurrection 1/day for free still seems quite powerful to me for a PC, even though that is what the template says. I'm in a generous mood today so.... I'd say, let 'em have the 1/day, but enforce the material component as usual so it isn't TOO easy.
There is a lot of stuff going on here, so balance is a bit hard to nail down precisely... I can't quite make up my mind whether it's weak, balanced, or strong. Three immunities, decent ability buffs, and unlimited flight are quite strong.
Compared to a dragon disciple, this class has more abilities, but it does lack the potent breath weapon ability -- so overall I'd say it's roughly equal to Dragon Disciple. So in this sense, I'd say it's balanced.
The confusing thing is that it lacks real offensive powers, any real combat feats or special abilities, and a great hit die. It also lacks spellcasting advancement and the spells are fixed, so it seems to lag behind the cleric a little also. So in this sense, it seems a little underpowered. As you said though, this class is more for roleplaying/story purposes than it is for power and abilities.
If you do feel the class is weak (and I think it's possible to make a case for that), you can add something small to it. Maybe bump up detect evil to at will instead of 1/day (given the strong connection to good), or maybe something like the Divine Grace or Lay on Hands paladin ability.
Putting the analysis completely aside, the simplest test may be the best of all. Stepping back and looking at the whole, I can't really say that I'd be afraid to let this in my game, so I would say no, I don't think it is terribly unbalanced, even as is.