My players want to take on the nation of Geb and want to create characters that are specialist in Undead Hunting. Where should I look for prestige classes or just feat and spell list that would be most helpful?
im not sure how much this helps you or if you like converting certain things for PF, but in the Midnight campaign setting, i believe in the Against the Shadow book, there was a prestige class called the Fellhunter, which was all bout killing undead.
if you google Midnight rpg fellhunter prestige class you may be able to find it for free (i found a lot of midnight stuff for free online) and someone may have even converted it.
Clerics, Ranger with favoured enemy Undead or Wizard (maybe Necromancer). From AGP Paladin Undead Scourge, Oracle of Life (or Bones... but that is kind of like fighting fire with fire... probably not what yo are looking for).
Some Necromancy spells are very effective (and only effective) against undead. So that is the category you should look into when selecting spells.