A pre-fab metal building would be a good option I think, but then again so isn't a stick built building or pole barn. The problem I find with the pre-fabs is if you are looking for budget friendly, they are usually long but narrow. When you increase the width I find the pricing to increase dramatically as it requires heavier cross beams to support the weight. Also, you'll need to figure in snow load which is sometimes a local requirement (regardless if it's pitched or metal roofing). I guess it depends on budget like everything else, but I will say like everyone else, width is just as important as depth.
As far as the floor, if you can afford concrete, go with the concrete especially here in the north. Hands down you will thank yourself. Not only is it the ultimate moister barrier, it will also enable you to sweep it, control oil spills/leaks, use a floor jack, roll around on a creeper, etc. I have heard of some people putting up the structure, then having the inside hot topped with pavement. My biggest issue with this is the smell, ventilation, and the pavement being soft. Although, I have only seen this done on fabric type enclosures. It is much cheaper than concrete and will provide a decent moister barrier, however not anywhere near as good as concrete. Hope this helps.
