If you're putting wood down as the floor, don't put it right on the ground. Our shed is wood from the bottom to the top and planted right on the ground. The wood on our shed wicked water up from the ground over the years. The floor is almost entirely rotted out on one side and quite 'bouncy' in the middle. There's also all kinds of pretty colors of mold growing inside.

I plan to tear the old shed down some day and replace it with a plastic one. (The plastic ones look better than the existing one. I don't need anything more to hold rakes and a lawnmower.)

I was planning on laying a gravel pad for the new one and possibly putting it in a higher location. That's just my 'draft' theory. Revisions to be expected!

Check your local building codes. In my area if the shed is erected on a slab it is considered a permanent structure and requires a building permit. If it is on a gravel bed or raised on blocks it is considered a temporary structure and no permit required.

3/4" modified stone(ie. gray crushed stone) is usually used around here for putting under a shed. I don't really think the $250 type of shed from Lowes needs a slab, that would probably cost more than the shed itself.
You can use pressure treated 4x4 right on the stone base, and then use 2x4s perpendicular to the 4x4s