Trying to come to a number

What ever the number is, put everything in writing and have both of you sign duplicates pieces of paper saying exactly what is to be done and what the agreed payment schedule is to be. I made the mistake of trusting a car of mine to a member here and he has had the car for almost 3 years and has barely touched it but I have paid him way too much in advance. I’m not sure what to do about it. I will probably not see the money that I paid him and I have three parts cars at his house that he was supposed to use to help with the building of my car, Most of which are missing parts that were not used on my car. Don’t trust anyone.