If you do not require it all to be 100%
GW models go with the avatars of war warriors in resin. Cost slightly less than
GW too and can be ordered with specific weapons.
If you do require them to be
GW models it gets trickier.
Converting them might be an option, or at the very least using conversion parts which will up the cost. Spartanesque shields and helmets for example will give a regiment a different look, bit of green stuff for cloaks/robes perhaps.
Easiest way if you wanted to do a fully converted army would be a nurgle theme probably.
Perhaps mixing in some of the older warriors would appeal to you too, theres usually some on ebay, plus those ones come with halberds if you get the 6th ed boxes.
All cavalry is a bit iffy still I think, but
WoC cavalry is still good. Marauder horsemen for warmachine hunting/redirecting and chaos knights still currently pack a punch nicely. Mine have luck going after
BSB units and running them down, usually on the charge for some odd reason

.
The new book is rumoured for this year so if you go the conversion route I would advise starting now and playing with the new book as the army will be well underway by sept/oct

.