A word on using straw as bedding, particularly if using the deep litter method. Straw is an ideal bedding for large/dry animal manure such as horses. It makes a firm "floor" under the manure pile so that a pitchfork can pick up the whole pile of manure, plus straw, and pitch it all in one scoop.
Exactly what makes it ideal for horse manure makes it terrible for chicken manure which is small and wet. You'd get a compacted layer of straw which the manure would sit on top of.
The point of the deep litter method is to have the manure constantly scratched under by the chickens. When fresh manure is scratched under the smell is greatly decreased. So, you want a bedding substance that is small and light weight. I use grass clippings and leaves from my trees because I'm cheap and you can't beat free. Ideally I'd run the leaves through a shredder to make them smaller but I don't have one and it's working fine. If you had access to cheap straw and a shredder this could be an option to make bedding that was more easily turned by the chickens.
Another purpose of the deep litter method is to promote healthy organisms that both compost the bedding and improve the chicken's digestive health, as well as provide some heat during the winter as naturally occurs when things compost. So, you want something like an active compost pile happening in the floor of your coop. For this reason I wouldn't use sand. You've done a huge amount of work digging out dirt you think was contaminated--now you're down to dirt that will be just fine as a base to begin your deep litter upon.
All this to say that I think you can save yourself some money by not buying sand or straw. (Not that they wouldn't LOVE a small sand section to dust bathe in!) I think you mentioned an existing compost pile on the property? I'd move this into the floor of the building to "seed" it with the microorganisms that are needed for composting. Then, I'd start adding lawn clippings/leaves as I had them.
Last thought: you want to encourage the chickens to scratch in their bedding so as to turn under the fresh manure and add oxygen to aid composting so plan to throw down something like kitchen scraps or BOSS or mealworms on their bedding every day or two. (I use kitchen scraps--can't beat free!). If using kitchen scraps I recommend limiting it to only fruit/vegetable items (no meat or dairy) just as I'd put into a compost pile but many people disagree with me and throw in anything.
Good luck! I built my own coop with no help from my husband as he was busy elsewhere and it's still a source of pride for me!
