Keep an eye on Stepper Motor shaft speeds. As a general rule pushing any step motor beyond 1.0kHz full steps gets you into a fairly low point on the torque curve. And at 1.2kHz the P43G would have to make 3k RPM just to realize 0.4°/second slew speed.

Going to 24V provides 4X the Watts with a rather modest current draw, which is fairly typical of tin can motors of that era. 

A typical modern NEMA 17 might be rated at 1.5A and would draw about 0.8-9A and produce about 30 Nm.  2.05714  W converts to 2.06Nm. Ignoring frictional and EMF losses, the 250:1 gear head multiplies the motor's output to 515Nm (0.7HP). That would seem adequate for any reasonable payload. As Khalid likes to remind us the 200" Palomar on has a 1/4HP motor for its motion.