Clean up and fix previously written code

I have a joomla component and related code that was written by a shop that needs to be reworked and optimized. The code interfaces to a 3rd party system via API to gather information. The application is supposed to create invoices in the component (and its related tables), email them and allow management of those invoices (change status, resend, mark paid, etc). On the frontend, the component should integrate as a module of community builder. currently there are issues with ..