dated and mileage when it was done... lower the price... with a 10% down payment agreement that states that he needs to get a full detailed inspection on it at a good reputable Z mechanic in which if there are any problems he needs to fix it... If its too bad then you can abort and in the agreement say you can take half of your down payment away... Gives an incentive for him to sell it to you and shows that you're serious.
Something to that effect. Then you have a substantive contract and some protection from contract law. 1. offer is made, 2. acceptance., 3. $ exchange...