I would agree that PFSense has all the capabilities needed to give priority to your VOIP traffic and has a fairly steep learning curve. I suggest using the simple features first to somewhat flatten the learning curve. A simple priority queue with VOIP, web and FTP priorities would probably do fine. Also, I would benchmark the thin client before routing all of the traffic through it. I updated my hardware when Comcast increased their peak download speed to 15 Mbps and my hardware was limiting it to 5 Mbps. Much better now

Enjoy!