Coming from a construction background, I've built a number of sheds for my motorcycle-building friends, and the most important thing...already addressed, but I want to reiterate it..is to overbuild your foundation and floor...
This is where a lot of the shed kits fall down, with their idiotic suggestions that you just plop the floor on the ground, or on a few lame 2x3's. This is just my personal preference, but I don't want any siding within 6 inches of the ground, so I essentially build a platform and put the shed on that.
You want either PT 4x4's, or, usually with the sheds I built, perimeter foundations of PT 2x6's bolted to PT4x4's in the ground (either with or without concrete , with 2x6's every 16 inches. It wouldn't even cost that much to make the platform out of 2x8's.
For the floor, I'd use two layers of plywood, probably 1/2", or maybe even 5/8", with overlapping seams...you only have one chance to make the foundation and floor, and you're going to have some heavy equipment inside...springy floors and siding in contact with the ground just add up to a shed that will rot away and be miserable to work in...