Do not assume that because the previous humans had no health issues from their chickens living in such filthy conditions that their flock wasn't carrying an infectious disease. Most avian diseases do not cross over to humans. The disease in my flock doesn't make their eggs unfit for human consumption, not is anyone, including me, at risk of contracting it. What does happen with a disease such as what infects my flock is that any new chickens I introduce into my flock will be exposed to the disease and if they don't develop a resistance they can die from it. Some diseases, such as Marek's can live in the soil and in the infrastructure for decades, infecting future flocks that you install. There are no antiseptic cleaners that can eliminate these very stubborn viruses. If you choose to ignore this danger, it's your decision, but there are simple soil tests that will give you peace of mind. Ask that they test for avian viruses. I would absolutely shovel out the mess, vacuum the dust, and wash it down with a bleach solution or Oxine. Then I would get a good premises pest spray such as Elector PSP, and thoroughly wet down the entire coop inside, getting into all cracks and crevices. I would also take a thin blade and scrape deep in a few selected crevices and see if there are any poultry ticks living in them. The spray may not kill those. After you know the premises are free if disease and parasites, then you will have peace of mind and can introduce a new flock and enjoy them free from worry.