Marauders on Foot and on Horseback are really useful. Chaos Warriors end up being really expensive points wise. They make great screens for your frenzied Knights.
Nurgle magic, especially at low level is good stuff. Also don't discount the Palaquin as a mount for them, 6 Poison attacks aren't bad.
Slaanesh is a GREAT mark because of how much in the game causes Fear/Terror. Also its cheap.