1) Is the Sunfire spell actually any good? I just got my [censored] handed to me by a vamp scout party and sunfire seemed pretty lame compared to my normal fire destructo spells

2) Can I buy more armoured trolls ad infinitum when my current one kicks the bucket?

3) Can my kids adopt a troll?

4) How do I cure Serena, whats the dialogue option I need to take/avoid?

5) Can I marry Serena?

6) How can I buy/get one of those husky dogs in fort dawnguard?

5) Can I marry Serena?
Nope.

6) How can I buy/get one of those husky dogs in fort dawnguard?
Once you return to the fort after recruiting the two NPCs Isran sends you to find, you'll be able to simply tell one of the dogs to follow you and it'll do so.

Dunno about the other questions.

1. I don't know, haven't used them much.

2. I guess so.

3. I don't know, might get cramped in Breezehome though.

4. Not 100% sure, but be nice to her and pick warm and fuzzy options.

5. No.

6. Progress in the Dawnguard questlines.

http://www.uesp.net - this site can help you a lot

Sunfire and Vampire Bane can be useful when dealing with vampires mixed in amongst friendlies. Those spells only effect the undead, so you can toss either of them at a non-vampire NPC and they won't do any damage. So, for instance, if you go to Riften and the entire town has some vampire surrounded where you can't get at it without hitting someone, you can toss these spells and they will only hurt the vampire, not the other NPCs.

1) Stendarr's Aura + Vampire's Bane - both these spells are a blessing.

2) Yes

3) Yes apparently (though I never would, heheh...)

4) Don't even talk to her about it until the main Dawnguard quest is done. Always ask only about her well-being.

5) No.

6) Dunno. But they'd make a better companion for the kids!