You seem to be on the right track. You will need to authenticate each and every dollar of the $500,00 investment. Bank records, payroll records, investment summaries, and declarations can be used. As you so correctly state, using a lawyer will help the process immensely.
The assistance you need regarding the relevant EB-5 documentation will require you to retain the services of an experienced EB-5 attorney. The preparation of an EB-5, I-526 Investors petition requires more than the documentation you refer to in your Question. It also requires the information and documentation for the project to be EB-5 compliant.