Dear Dodgeman....... Even tho you are building just a garage, you are building a substantial "structure". Substantial structures need a "footing". This is very much true in an area that gets the frost you described. The size of the footings depends on the loads you are going to put on it. I assume by "floating" slab you meant to just put in a slab, with the thickened perimeter, in some attempt to not install "footings".......(there is that word again). Don't do it. The forces of nature are against you in this instance. Sandy soils like broad bearing surfaces.....which to your credit you are considering........but I wouldn't risk substantial monies on the freeze/thaw conditions you are dealing with.
Personally, I would consult a structual engineer or at the very minimum, a "very reputable" footing and foundation contractor. They will know what to do. In the building world, there are a few areas you don't "dink" with.......you got it...........footings. Take your wish list to the man, my man.......and you will sleep a lot better.