Never pay 100% of the value of the property until you are sure all is ready. Keep at least 10% until the very last day! I have seen many times, that the contractor promises so many things before sale and once he is paid 100% of the value, he forgets half of these promises!

Always make a contract and hire a lawyer (it will cost extra but its worth it). Your lawyer should be someone who is not affiliated with the contractor. It is funny, sometimes I see buyers making a contract and the lawyer is contractor's friend and paid by him. If a dispute happens, the lawyer will be on the side of the contractor not the buyer. Be very careful!