lichick,
My girls have a concrete floor in their coop. Actually it is marble mosaic covered with straw wall-to-wall carpeting. I like that it is really easy to sweep out and hose off. The hens don't really care for it as much because they can't play in the deep litter the way they used to. And they never dust bathe indoors, they use the covered patio for that. I think this keeps the coop cleaner too. Around here they like to be outside almost all year, so the run is the most important part.
We don't have foxes here, so I am not sure of their habits. If they are purely nocturnal hunters you can limit your fox proofing to the coop if you secure it at night. I prefer an automatic door that is farmer-proof.
I would think that if you wanted to keep foxes from digging into the run you could do it in pretty much any of the ways recommended. The key thing being a fox-proof barrier. For something as big as a fox you could use fox-sized stones around the run arranged on or near the surface that as other experienced people have suggested would be either fox-deep or fox-wide. Urbanite, or broken concrete slabs, are very cheap and easy to come by here. You could just lay a couple layers of it around the run, and put enough dirt on it to cover it if you want.
Any concrete you pour in the ground will crack and heave if it is not below the frost line. In Minnesota the frost depth is 48 inches, so poured concrete is never an easy option. This is especially true if you put a fence footing on top of the concrete. A set of fence posts that shift a couple of degrees off plumb every winter will soon create something other than a fence.
Also, adding a foundation to an existing building is a lot of work. If you look at my coop you can see that it is a little slanted after being jacked up to make room to pour the floor.
The other consideration is the surface material for the run. Because I do not have the acreage you do I have to recycle it all. The brush and leaves and weeds go into the run and the compost comes out. Since I do not want to scrape up gravel and put it in my garden I just dig into the clay an inch and then throw in as much vegetable stuff as I can get my hands on. When it is dirty (smelly) I rake it into the compost bins until it turns into a worm salad for the chickens and then black gold for the garden.
Good luck, and much happiness.