There´s only one kind of unit I´ve been hiring as much as I could in vanilla MTW, and that´s Druzina cavalry. Why? Because they dismout to 80 Feudal Foot Knights anytime and those make a hell of a killer unit in early (when the Chivalrics arrive the FFKs loose some of their shine but still are excellent swordsmen). Anytime else, I´ve only hired mercs if I needed to boost my ranks quickly.