Pretty sure Moleman's in USAnia, where construction practices may vary ...
I'd expect there to be a damp-proof membrane in the floor, but anything's possible. Is this garage part of the flat building, or in a separate block? How well insulated are the roof and walls?
Yes, I plead guilty to residence location as charged. And, my long-windedness about commercial flooring failures over here comes from work as an architect and seeing grocery store flooring failures by the acre.
We have a garage which always has a damp concrete surface when it is moist outside (and, near Seattle, that is often) due to sub-slab moisture, because the contractor and concrete workers couldn't be bothered to put down the layer of plastic that my drawings clearly showed.
The "hang it from pulleys" idea was often done over here, by those who shared their model railroad space with a passenger motor vehicle. Growing up, my same-age neighbor's Lionel 3-rail trains operated on a flat plywood prairie which was suspended from the garage roof, and had fold-down legs. This would have the benefit of getting the layout up to the warmer (hot air rising by convection) area of Martin's garage. The plastic cover idea, suggested elsewhere, could mitigate the inevitable accumulation of dust.
Can you rent a dehumidifier for a week or so and see how much moisture it collects? That would give an idea of what capacity/rating to purchase, or even if it's a good idea; if possible, a unit that just drains to the outside would solve the "did I, or did I not, empty the collection vessel?" question. Best would be if the trial week were a wet one outdoors, and not much water was collected ... would suggest not too big of an on-going problem.
You likely will want some heat in the garage when you are operating, of course.
