Ok. I'm not going to argue. But if you can't afford substrate for a few weeks...
What are you going to do if the animal needs a vet?
What are you going to do if heating equipment fails?
What are you gonna do if the thermostat fails (although based on your description of the previous owners setup I doubt there is one)?
Taking on an animal that was already poorly cared for drastically increases the odds of needing a vet or equipment upgrades.