Take a used small form factor PC that has at least a 2.4Ghz Core Duo processor, 2GB of ram and two Gigabit Ethernet LAN ports and install PFSense. You won't be sorry! Simply, quick, inexpensive and manageable.
Plenty of performance and the ability to turn on VPN (OpenVPN) access to you can remotely manage your customer's needs. Try using the Viscosity VPN client ... Mac or PC, you will love it!
(Don't forget to contribute to the project!)