How to combine maintenance visit, timesheets & items into 1 document with create invoice link?

We currently use what we call a service work order to do work at customers buildings. Upon completion of the work, we need to complete a service work order to document what work was done and record the items used (without pricing), hours spent on site and any other related notes. After this we need to have the client sign the service work order.
Once back at the office, we take the work order and create a sales invoice for the hours and items/parts used and then send the invoice and a copy of the service work order with the invoice to the client for payment.
Looking to be able to do this all in one module/document with a link to create invoice from the maintenance visit document.
I started to modify the maintenance visit document to add the other fields, tables etc. but got stuck.
Looking for the easiest way to combine these things. Is creating a new module the easiest way to go or is modifying one document better? Any other advice is appreciated.