Why not just make favored enemy bonus equal to a ranger of the dragonslayer's Hit Dice?
Favored Enemy (Dragon) (Ex): Undead dragon slayers have dragons as favored enemies, as per the ranger class ability. The bonus is equal to the maximum bonus achieved by a ranger of the dragon slayer's HD.
Hmmm, if they were already rangers concerned with dragons, shouldn't it be double good? I guess that does sound over-powered. Makes sense.
Sorry, I meant not to allow stacking.
Good. How about the casting and dragonbane bits?