Now I don't know much about a witch hunter build, but if you plan to defeat mages, I would say start with a Breton. He has 25% magic resistance from the start, plus the 50% absorb magic daily power. You can combine this with the Atronach stone for an additional 50% absorb magic. That means for 1 minute a day you are immune to magic, that sounds pretty good to kill mages with. You can do the Agent of Mara quest for 15% magic resistance, get into Alteration for the Magic Resist perks, and get into Enchanting to make your own magic resist gear.