[caladina]

how about mounting the shed base on 6 or 8 shopping trolly wheels or simalar,
have a motor driven wheel to turn the shed, obviously it will need a track or guide to keep it in a circle

also as you probably know will need a input for the motor to keep it in time with the earths motion if doing long exp pics

maybe you can get just the motor drive from a proper obs'

have you looked at components for proper motorised obs' ?

mind you if you not worried about keeping the motion of the shed its self in time with the earth then you should be able to turn it with and 240 motor or even manully

shopping trolly wheels are fantasticly usefull, i use them for moving smashed cars, even with the engines still in

personally i would leave the shed base fixed and have the seating position along with the scope and roof / dome that rotates,
if you get an iron works to make the large circular track for the roof / dome to rotate on the rest you might be able to do yourself