I have a real nice pole building hunting cabin (nice enough to be a home) just like you describe. It is 35' x 55', 825 SF is garage and 1,100 SF of living space; 2 bedrooms, nice bathroom, large open kitchen / dining / living room area and a snall mechanical room. Kept it simple to control costs; exposed concrete slab with throw rugs, OSB plywood ceiling, a single thru-wall electric heat & AC unit, etc. I insulated the heck out of the place, put in a nice woodstove, and made the kitchen real nice. I've got about $110,000 in the place complete including the septic and utility hook ups, land cost was extra. I like it, almost zero maintenance and my only cost is about $150/mo electric, $22/mo water and land taxes.
Note - You don't need a 6" slab for a residence or the garage, 4" is adequate.
Note - You don't need a 6" slab for a residence or the garage, 4" is adequate.