Although it won't do what you're asking out of the box, I second the recommendation of ClarkConnect as a firewall. I use it at home and used it at work until the hardware died.

To acheive true privacy you'd need separate ethernet interfaces.
Clarkconnect does do squid logging i.e. web proxy only. But it does not do logging of all traffic.
CC doesn't do any sort of accounting/monitoring/notifications that you are asking for.
CC has the bpalogin client for Telstra broadband as an option which makes things easy.

CC is linux based though so it's possibly a good starting point but you'll need to put in a bit of work yourself to reach your goals.

BTW: Telstra slows to 64kbps after 10GB. Optus slows to 28kbps.

There might be a better suited distro, but when I was looking I tried almost everything. I used Smoothwall intially but it's just a basic firewall/router. It doesn't have any of servers like mail and web like ClarkConnect.

I really didn't like Astaro, SME, Mandrake firewall or any of the others I looked at or tried.