If you put plastic under the dirt the track will not drain well. With only 10 loads, the dirt base will be thin. I would get a Skid Steer and strip off the grass layer and see what type of dirt is natural. Clay is not a requirement. I've seen some very good top soil based tracks over the years. If you are purchasing dirt, ask the contractor if you can go and see the dirt first. Make sure it's mostly rock free before commiting to purchase.