A2Billing, Asterisk, FreePBX on CentOS VPS for WISP

As a small WISP (wireless internet service provider) we would like to start offering VoIP to our customers. We have tried the billing systems and SIP registrars of offsite hosted ITSPs, but it does not work for us. We want to install Asterisk + FreePBX + A2Billing on our own hardware running XenServer 5.6 and CentOS 5. It has two network interfaces (eth0 to the internet with public IPs and eth1 to our internal network with private IPs). We need to set it up this way to avoid NAT problems experienced with SIP. We are currently making use of a single ITSP giving us SIP to their side with multiple DIDs for incoming calls, which we will in turn assign to specific customers. We need to be able to configure our own billing/rates (different for business and after-hours) to bill customers. Monthly "line rental" should also be taken into account. Customers should be able to register to our server using SIP or IAX. We will do our own customised branding if you can show us where the images and HTML files for A2Billing are located. Basic training/instructions on where to look when something goes wrong would be essential. A basic backup of MySQL, configs and HTML files should be set up on a cron job.