Getting sued by former property management company?

I'll try to make this short. I had 3 properties in Illinois managed by the same company. The properties were crap, communication was always awful and I finally just unloaded them at a loss. During the 3 years I owned them I paid for things to be fixed that I later toured the properties and saw that they were not.
After I sold the properties I gat an invoice for over 19k for things done to all 3 properties. About 5k worth of stuff I believe was done (hot water heater and some electrical needed to get one rented and it was rented so it was likely done) The other was for stuff he out in a home to work on but was stolen, flooring, clean outs... Stuff like that. I have a contract I signed with him stating anything over $250 needs to be approved by me (nothing was). I looked at the contract and he didn't sign it (but it was provided by him) I live in KY and no longer own property in Illinois. How hard would it be for him to sue me and how likely is it he would win? This was last December and I asked for invoices and never got any. Now he is calling and texting nonstop threatening to get lawyers involved if I don't pay.