Basic Servo Motor Question...

I want to jump in head first in a CNC servo sherline mill conversion,
but I am really ignorant when it comes to DC servo motors and
drivers.

I see that some servos have slotted encoder wheels with optical
detectors and/or tachs and/or brakes. I can see what the slotted
wheels and brakes do... the tell position and stop the thing, but
what
is the need for tachs? Can't you tell position AND speed with the
optical detectors? What motor stats are important for me if I am
going to use a servo motor with the sherline? Does stall current x
volts (watts) tell me something? What RPM range is good?

I was browsing a local surplus store this past weekend ran across a
servo motor with one of those slotted wheels and an optical detector
circuit board attached still in the anti-static wrap. It was made by
Japan Servo and it's rated at 24v 3000 RPM. It is about 3" in
diameter and 5" long. I don't have any other data on it. How can I
determine if this motor is a good candidate for the sherline? At 12
volts it really seems to have a good bit or torque.

Thanks in advance for any answers... Pointers to "servo CNC motors
and drivers for dummies" websites would also be great!

Thanks
- Dan