Your torque requirement will depend of course on the load and what kind of acceleration you need, It might be an idea to use a sizing software of which there are several free ones http://www.electromate.com/technical...morgansoftware
The common industry standard is to aim for a motor size that keeps the inertia ratio of motor to load of less than 10:1. This is subject to load and acceleration.
There are zero backlash couplers out there, one of my favorites is one made by Lovejoy/Rosta 'U' series, which resembles two coil springs contra wound, one inside the other.
What type of control system did it have before can you reuse the motors and add new amplifiers?