There were no special requirements for mercenary barbarians, but all hailed from the hills, jungle, or plains.
Those who didn't have slave warriors (mamluks) often hired mercenary barbarians to do their fighting for them. As such, these warriors were considered a necessary evil. These warriors were awkward when associating with anyone not of their homeland. They were often strong fighters, but not as adept at fighting as a single unit as mamluks were.
Mercenary barbarians had an unlimited amount of choice when selecting their equipment, as long as their purse could back up their selections.
Their bad reputation hindered these warriors when dealing with all not from their native homeland.