I would not hesitate to use such a slab as stated. As additional protection a protective layer of 2" foam around the perimeter, laid flat, and extending 2' away from the building, 4' would be better and the experts engineer call for 6'. The soil should be well drained. If you have a soil which retains a lot of water or the site is wet, a slab is not the best bet. Drainage will be important. Gutters help control water flow. If you get lots of snow this helps insulate the ground as well. I would refrain from plowing near the sides of the building, as this surface would be exposed to night time radiation and more frost build up.
I have two building a 30'x60' and a 28x36' both on slabs with less thickness on the edges than mentioned and they are functioning well, and no insulation around the edges. One will receive the foam around the edge this fall, it has been in service for near 10 years now and we are about to move in.
One has 8x8 sills, the other a 2x10 plate.
Tim
