Yes, -40C (this is in Northern Finland); we don't believe there's any reinforcement in the slab. Pouring on top of the existing slab sounds attractive, I need to check that the current garage doors height will still be sufficient if we raise the floor level; that may also limit the machine I hire, a 5 tonner is quite high.
I suspect that the building contractors are covering their behinds by quoting for a completely new slab so they can be 100% certain of no future issues; I am not worried if a few cracks appear somewhere down the road!
Yes, we are (heavily) insulating the barn and installing heating, after all my new man-cave needs to be cosy!