Our Masterwood point to point has 2 9HP HSD's. One rarely gets used, but the other one has been working fine for about 7 years now. Whether a 3HP is adequate depends on how rigid your machine is, and how fast can it go? How deep do you want to cut in a single pass?
I thought most spindles have relatively constant torque? Depending on what tooling you'll be using. most likely you'll want to use a wide range of RPM's.