Sounds like you could be looking for something like the Sonicwall TZ 100, which is a firewall router that I've recommended before on here to several others, and I've installed and set up several small business locally with them as well. These are great devices, low cost, and have all the features you are looking for I believe. What's great is how easily Sonicwall firewalls can be configured into site-to-site VPN tunnels. You don't even need static IP addresses on both sides of the tunnel, only on one end. The Sonicwall firewalls also come in a pretty good range of capabilities, so if you have one office that is larger or has greater networking demands than the others, you can always get a slightly more robust model and everything still works the same all together.