What is the best firewall distro/application for SOHO users?
I recently had an idea to provide the following service to small businesses: Create and manage a basic firewall/gateway that could handle content filtering, bandwidth throttling, and firewall services(anti virus and the like).
The idea is that, especially where I live, small businesses really have no implementation of network security, or really even a concept of it. My thought is that I could provide this service to them for a flat monthly fee using relatively cheap hardware and an open source linux platform/application. I could manage it almost entirely from my house through webmin, ssh, or some flavor of remote management, and I could potentially handle large amounts of clients by effectively scripting the entire process.
That being said, I would like to ask one specific question here. What is the best distro/application to use in this context, and how could one practically go about implementing it? My knowledge of linux has been exploding as of late because I have been forcing myself to use it on all of my machines instead of Windows, however I can't seem to find any clear answer on the first half of the question, let alone the last half.
Any foresight, knowledge, opinions, facts, instruction manuals(especially instruction manuals), or insight would be extremely helpful. I am here to learn, so let loose the hounds of knowledge!