It depends on how fast you want to go. The SMALLEST motor I would use is a Porter Cable 7518. http://www.porter-cable.com/index.asp?e=547&p=2819
At a certain speed, though, even this will not have enough power. Unfortunately, for the next step up, the price goes up at least 10x.
Look here: http://www.ekstromcarlson.com/router_packages.htm
Our 12HP router at work can cut 3/4" plywood at 10m per minute at 14,000 rpm, if that helps you at all. Tooling plays a big part.