Sand can actually be a poor choice in wet soil. If my pen is really (I mean REALLY) muddy I will sometimes pitch in some straw for them to walk on near the ramp, helps keep the eggs cleaner. And I have also used some wood shavings as well, same reason. I leave whatever it is in and it just...
Yes, you will need bedding as well. Here is a Learning Center article on bedding
https://www.backyardchickens.com/a/quick-guide-to-common-brooder-and-coop-bedding-materials
How big is your coop?
My elevated coop has a plywood floor covered with a sheet of scrap vinyl flooring - durable, moisture resistant and easy to clean.