Gus, What about using the "bottom bracket" from a three piece assembly (mid level bicycle) and the associated gearing from the back hub as well as the revelant parts of the frame?
The bottom bracket would form the carry through shaft of the elevation rotator. Fasten the seat post to a plate and so on.. Could reuse the fork assembly as the azimuth bearing.
The motor and gear ratios are just scaling factors in equations.
One of my first instincts was to go after bicycle parts. This stuff is ubitiquious.
I suspect that it may be possible to find a new encoder of the correct type with a mount for a few dollars. One that is shaft mount, with an appropriate output and no detents. It might be the one bit of the project fit for a group buy. Norm n3ykf