How large does the entire slab need to be and will there be anything else placed on it?
Will this be indoors or outdoors? If outdoors, what region of the country are you in? Will frost heave adversely impact the use?
Great. I would make this a 12" thick pad for the 10' diameter circle. You can use regular rebar, but epoxy coated would be recommended due to the exterior application.
I would also remove the 5" of topsoil and place this on soil that is better suited for bearing such a heavy load.