Let me be honest,
The best option is to find some freelance who already have the same module and, inside that, make the changes. Otherwise, other alternative is to personalize a similar module. In my case, I have a Pet Attendance (Veterinary). The relation between Customer/Pet and Pet Attendance is the same of Customer/Vehicle and Vehicle Repair, respectively. I have a partner in Mexico who have developed TMS and MRO modules and implement in more than 12 companies. Unfortunately, I can't ask him for the code but he can help in order to power this and execute the project. With this, I think I've justified the quotation I present to you.