How do i know there is no other debt 50% is a large amount too keep on your property and how do I know someone is not going to make a claim later or if the guy will even move out?
The notary is responsible for making sure that the debts are cleared. Discuss it with your chosen notary.
Making a claim later is irrelevant, as they need to lodge the claim against the property while it's still in his possession. If they come later, there's a simple legal device that states that he doesn't live there.
As for him moving out, simply require him to deregister himself from the address register before you visit the notary and make sure that the keys to the property are in the hands of the notary before signing the contract.
If the debt is attached to the property rather than the owner, then... don't get involved.