Before deciding on encoders, ask yourself why you need encoders?
In OnStep, encoders is for pointing, and not for tracking.
By pointing, this means that you can do things like: release the clutch,
move the tube then engage the clutch and OnStep will still know where
in the sky it is pointing.
Without encoders, OnStep will still point properly, provided that all
motion is done from OnStep's Goto. But it loses track of where it is
pointing if you disengage the motors and move the tube manually.
Encoders are not for RA tracking accuracy though.