For a robot arm I would focus on implementing two joint ik systems for segments.
It also depends if the plc controller would pass in specific motor rotations,or if would it just be start / stop / rotation direction signals.
You also need to identify the protocol of the PLC (does it use CIP).
Does it communicate via rs232 / com / lan etc
Then build a plugin to bridge with the protocol (probably need a handshake to establish connection) and then translate the information to unreal.