I have a friend that built one similar to what your building, He used some of the wheel motors like a Power Trac from Surplus Center. He used 15 inch tires, He made his a copy of a CHeetah Roofing loader.
http://www.tigerequip.com/images/pdflowcheetah.pdf
He made a seat and a small cage for his, He used Cheetah and the Avant skid loader as an example.
Now thats what I'm talking about ! I like where you are at with those models.
Thats about the size I have in mind.
From an alternative means of power distribution,could a open deferential with "cutting" brakes be used in said unit instead of hydraulic motors ? Each side would be chained front-back.