Currently our home network consists of a wireless ADSL router, 2 desktop PC's and 2 laptops connected by wired and wireless connections to the router.  I wish to build a central server machine as I have a spare older machine not being used.  The main purpose of the machine would be to control the bandwidth usage of my flatmates and I so no one uses an unfair amount of bandwidth and so no one can hog all the bandwidth slowing the rest of us down.

With our ISP we can buy monthly blocks of data and we typically buy around 75GB a month.  I was wondering if there was a way to limit this to around 15GB each and then either slow that users connection to the Internet to around 20KB/s for the remainder of the month (without slowing their local network access speed) or a way to track and record each users bandwidth usage throughout the monthly period so that we can work out our fair share of the bill to pay?

I think the setup would involve two ethernet connections on the server machine, one for the ADSL modem and one for a wireless router.  I would want to control the access to the internet being ethernet port 1 and not control ethernet port 2 being the home network.

Thanks for any input.