Use a crushed stone mix with nothing larger than a #8 in it. Compact everything! You want at least a 1' X 1' footer with 2 #4 bars and 6" welded wire mesh in the slab. #3 bar 3' on center is better. Use 4000 PSI concrete with crushed stone. Do not use round stone. The cost difference on the project is minimal. Keep all the steel a minimum of 2" from the edge of the concrete. Piers need to be a minimum of 2' X2' to do much. Keep in mind anything like a lift you may be installing and thicken the slab there. I would stay away from casting plates and use a 1/2" diameter tapcon. I'm a commercial general contractor.

Old enough to know better, too young to resist.