At this point, as you are moving into legal document territory, the best thing would be to consult an attorney for advice. You need to find out from him how to make a legal contract, what all you are eligible for in terms of compensation and any risks you might what to mitigate up front.
All the advice i gave is simply based on what i see when it comes to various health care legal documents.
Off the top of my head get an independent 3rd party to witness and sign, be as specific as possible when it comes to dates, times, payment type, and why you are being paid(for damages done to my, your name here, 1999 Trans Am on 7/x/2011 in the amount of $XXXX.XX. $XXXX.XX will be paid by, her name here with initial, in full by 7/x/2011."