Drujistan is just the name of the south portion of the Ilbars mountains and is believed to be haunted.
'A long day's journey southward, among mountains so wild and barren not even a goat could dwell among them, lies the country we call Drujistan.'
'Drujistan!' echoed Conan. 'Land of demons, eh?'
'Aye! An evil region of black crags and wild gorges, shunned by wise men. It seems uninhabited, yet men dwell there – men or devils. Sometimes a man is slain or a child or woman stolen from a lonely trail, and we know it is their work. We have followed and glimpsed shadowy figures moving through the night, but always the trail ends against a blank cliff, through which only a demon could pass. Sometimes we hear drums echoing among the crags, or the roaring of the fiends.'
L. Sprague de Camp and R. E. Howard, The Flame Knife
As for racial characteristics, I'd use Himelian hillmen for the Ilbarsi tribes and Hyrkanians or Shemites for the flatland tribes. Maybe find a way to combine the Vendhyan, Hyrkanian and hillmen into one race. Keep in mind that Iranistan is not a major kingdom by any means. Only once is a member from that land mentioned by name in a Howard story (The People of the Black Circle). All other mentions are from pastiches or from The Flame Knife, which was a Howard non-Conan story (Three Bladed Doom) that de Camp rewrote into a Conan story.