Many.

My shed was built by or for the previous owner, and is on concrete. A flat slab, so when it rains or snow melts, it runs under the wall and into the shed. Not good for the wood clearly, although as far as I know it hasn't rotted yet. It has the plastic foam under the plate but of course that's not the solution. I am still trying to work out what would be best.

If I was doing it over or even putting a prefab shed on concrete, I would build a low wall/foundation round the slab that the walls would sit on, at least 6" above the ground to control the moisture.

Looking at what you want the shed for, much as I dislike excessive use of plastic (although at least recycled plastic products provide a market for the stuff we should be recycling) those plastic sheds you see in places like Costco seem pretty good, and they have well-raised floors. You could probably also run silicone around the edges (I would put some sort of silicone/dpc/caulking under the edges of a prefab steel shed too) as a safeguard.

The height of the floor seems pretty good and should help keep things waterproof. You will want to ensure that vermin access is controlled, and holes are covered, since mice love boxes of books.

If you went with the plastic, follow the instructions but you may be able to go direct on the ground, or you could put down a bed of sand, or perhaps a low wood retaining wall, fill with sand/gravel, create ramp for access.