I have used a plastic grid for the base of my slowly evolving log cabin (which I don’t suggest you build if it is a rental property, as it’s potentially too long lasting, will cost more, and take you quite a few days to build on your own). The grids are really simple, but you do need to have a level and stable base on which to put it: I used 3 big bags of MoT aggregate (delivered by a lorry with a grab arm), but that’s for a bigger space. It’s a lot cheaper and quicker than concrete, and easier to remove should you relocate again. Some of these grids are bedded in with pea-gravel, some not.
The only thing with a shed is to remember to keep the exterior in good nick. Preservative, preservative and more preservative….