Hats off to you! This is no easy undertaking. But you have come to the right place.
You are on the right track with functions. You can have custom code (C#) inside a function. The main difference is that instead of outputting to a control like epiNumericEditor… you send it to the output variable of the function.
Yes, you will need an event and trigger in App studio to pull the value from your function and plop it into a text or numeric box. Use a function widget, and a row-update widget in the event. Setting the fields for these can be tricky.
Check out our work in this thread Add File to Line Comments in PO Entry - Kinetic ERP - Epicor User Help Forum
Good luck!