References first. Contract second. notarized. $6,000 payment made at completion. Set a maximum time to finish and if contract not 100% fulfilled , no payment is due and parts installed must remain. Monitor progress weekly. No contingency for hidden problems, either in money or time. Complete one project at a time i.e. do not permit total tear apart first. He'd be nuts to sign such a contract. Get a list of parts sources and quality of parts and his cost and nothing used. Demand a warranty, perhaps 100 operating (use) hours.