-Witchers are immune to disease, heal very quickly, and have great stamina and resistance to poison.
This says Dragonborn to me, though I don't know where you'd get Fast Healing.

Geralt, and Witchers in general, are highly knowledgeable about monsters of all kinds, herbs and potion-making, poisons, magic, and probably a wide variety of other things.
Could do with some Factotum. That gives you all Knowledge skills, and you could use Knowledge Devotion to get some use out of those Knowledge: Dungeoneering/Arcana/whatever skills.