Create Firmware/Software for my ISP company

I am looking to have program that can collect the data usage on a modem we provide our customers with. I want to keep track of how many gigabytes the customers are using and charge on a pay per use basis. I also want there to be 2 sections of this software. One side being locked by the company and inaccessible to the customer, the other being accessible by the customer so they can control certain aspects of the modem; quality of service and networking features, etc. There are several approved modems and several different internet connections that I would need to work with. I need the software to connect to a database that connects to our accounting software that allows us to bill accordingly as the data is used. In your proposal please explain how you can help me. Thank you!