Customize opencart to B2B functions

Hi all, For a new project we need to have a b2b store. In this case we choose opencart. (other optios are welcome! ) We need to add and disable some functions. 1. Remove self register option.( only customers can login if they add by us) 2. Edit or install a new invoice system so customer can see his discount on the invoice. 3. Remove shopping cart links in the header but must be vissble after login. This will be the first start.