With a 100-1 ratio, even a small harmonic drive will have plenty of torque for positioning a spindle-head or trunnion. What it won't have is speed.
Inaccuracies in indexed parts are usually due to improper zeroing of the rotary axis.
Yes, there are plenty of old rotary tables on the market; they used to be necessary for making round features, but modern CNCs are good enough to do that with G2/G3 commands. As long as your indexed toolpaths don't require changes of direction, the backlash in their gear trains won't be an issue. You can move it with a motor or a crank; it doesn't make much difference if you're monitoring progress and can step in to do it.