If im buying a truck 1st thing i look for is the service records.Whats been repaired or not repaired.If hes on with FECC he will have a summary of everything hes done to that truck evrery 6 months since he put it in service.You need to take it to a garage that can check the ECM for fault codes,this will tell you much about the engine,everything from actual fuel mileage to power settings,to any problems.As far as the reefer,you'll have to find out when it last was tval checked,the operating temps if they are ok,and also the ECM ill also tell you of problems.Along with the rest of what i've said,you want to check the drive train,u joints,any leaks,and tire wear is a give away if there is bearing wear in the axles.With just 300000 miles this may be a good buy,if the price is right. good luck