Not sure if this will help but I manufacture scale car components(I.E
engines,transmissions,differentials etc) and you could use a scale car front
end and add a gearmotor to turn the steering rod,or a heavy servo-I make them
from 10" across, up to 16" across. They are made from heavy guage aluminum and
can be screwed or bolted to a platform ready to go. They start at 25$. I have
used them on vehicles to carry more than 125 lbs. I can also supply heavy cast
aluminum wheels with the unit(no flats ever!).