In instances like this - divide and conquer! Divide the system up into manageable 'slices' (ie each segment of a few amps each) and have a psu per 'slice' so that you don't have to contend with tens of amps. The idea of using micros and the allegro driver is probably a good idea.I would probably use RS485 rather than 422 simply because of less wires and the interface consists of a 75176 or XX485 chip. Interfaces for the pc can be purchased easily. The ADVANTECH ADAM4561 usb->485/422/rs232 fits the bill nicely. The next hurdle is the protocol - years ago I would have cooked my own, nowadays I'd probably choose MODBUS ascii as it was mean't for plc control and would fit your requirements. Why? There's plenty of tools that talk MODBUS available (free demos even) allowing you to test the micro end of the protocol with a known source. Also using an known protocol means other people can talk to your device without you having to describe in length how your stuff works. Most SCADA packages know about modbus so that may make integration easier.

Your mileage may vary!