Watch the movie; Bram stoker Dracula and make a character based of Abraham Van Helsing, a true vampire hunter, he actually existed.
anyway, since its dawnguard it has to be
* archery
* one handed also works as a secondary weapon
* perhaps restoration? the good magic...
* armour choice is a little up to you, i'd go for light armour
* perhaps alchemy, real vampire hunters were known for being doctors
EDIT: Also, is it possible to begin and complete the Dawnguard quests at a low level starting immediately from escaping from Helgen?
yes, but you might have a hard time based on your prefered difficulty level