At least get one hellpit, its cheap point wise and used right can really put a dent into an enemy army. Also since its so huge and visually intimidating, your opponent will sink a majority of its force into it to try and take it down leaving the rest of your guys free to march on through. Slaves are a good choice to, especially if they are in a big unit and you ram them in and force them to scatter, does some nice damage from them fleeing. If used and equipped right a unit of plague monks is pretty decent.