Any place that sells pre-built sheds in your area would likely have advice on site preparation for delivery.
I don't know what the soil's like in your area, but if you're going to hit bedrock before 42", it'd likely be easier to put in a slab; If you're likely to hit lots of tree-roots, the slab might be a better choice (so you don't kill a tree, and have it fall on your new shed) For most other times, I'd personally go with footings, but I live in an area that's clay and easy to dig. (other than tree roots).
Some pre-built sheds are made to sit on a slab, as they have no floor; this can make it easier to get things in/out of the shed, as you don't have the step-up.
... also, rather than buying from Home Depot or similar, look to see if there are Amish or Menonite areas near you -- they often have businesses that do this sort of pre-built structure construction. It might be a little more expensive, but you can get them customized to a specific size, material, etc ... it's what I'd do if I wasn't going to build it myself.
You don't need to concrete the whole thing.
You do need to shape it correctly. Even in arid areas (seems likely from the picture), water is the major thing that destroys poorly built roads - when it does rain, the water flows down the road and moves material - unless the road is shaped to divert water off to the side in a short distance, so that there is never so much water collectively running on the road that it moves the gravel. In a first step this might consist of adding water bars every once in a while to divert water to the side, but a complete solution consists of putting crown on the road, so that rather than having two wheel ruts (which become water channels, and then become deeper ruts) you have a slightly mounded profile and all water runs off to the sides (and, if need be, you have adequate ditches to handle the water once it is off the side of the road.)
Depending on finances and inclination, you can do this piecemeal over time for little cash with shovels and rakes and implements of destruction, or you can hire a contractor with a road grader. If the contractor also has a rubber-tired vibratory roller and knows how to use it, so much the better. You may want to remove (or break into smaller chunks) the random chunks of concrete, unless you are adding sufficient material to bury them. You will occasionally need to re-rake as your tires move material, but it should not be frequent or major if you drive calmly and are not spinning your tires. If you allow tire ruts to remain, and it rains, you'll have worse ruts after it rains. An old bedspring or section of chainlink fence can also be used as a maintenance drag (vehicle-pulled rake-equivalent) to help keep things where they should be.
If your gravel is 1" down to fines, it should work. If it's 1" stones with no fines, it won't pack well - good for drainage, not so good as a road surface. A gravel/stone supplier should be able to get you a few loads of "road base" which will have enough fines to pack.
If you work on it piecemeal, work from the top down, so each section you complete will stay put.
Best Answer
CLR is what you're looking for. I had a similar problem as you (I was cutting metal on my driveway and did not sweep up the fines that night...Rusted the next day!) and it worked like a charm. It comes in a grey bottle with a multicoloured label.
Dilute using the measurements on the bottle, and scrub hard with a stiff bristled broom. Rinse well. This stuff is awesome elsewhere in the house too - in particular to clean up around taps.