Invoicing with SugarCRM - repost

I've been using a customized version of the SugarCRM Community Edition for a while. Now I have to expand the current version, and to add a new functionality consisting in generating invoices through SugarCRM. The job will consist into two main tasks: 1. to create some additional fields in existing modules and, eventually additional modules if required. 2. to import information required for the invoice generation from external xml, csv files (automatic process)