After further work it turns out that the MCP23017 may not be the best choice for my project, because of my limited programming skills.
Thanks to the good people at Adafruit and Joan form the Raspberrypi.org Projects forum - automation, sensing and robotics topic I believe I have found a solution to my RPi DIY motor control needs.
Long and short of it is:
DIY Ardunio motor shield @ < www.Instructables.com/id/DIY-arduino-motor-shield-L298N-Chip-2-4-Amp-/
> allows me to build the motor controller.
For help to get started with coding the shield go to <youtube.com/watch?v=qwNvYYqBy7I >
The young man hosting the video does a good job explaining H-bridges and the code he wrote to test run the DC motors being controlled by an Arduino motor controller.
The example code associated with the video can be found by clicking the "Show More" box below the video.
I hope this is helpful to those who might be looking for DIY solutions to bot motor control.
Thanks to all those who helped me plan for the RPi based robot's mobility. BTW the RMF (Robot Motor Factor) calculator at this site, along with constructing motor torque/speed charts played a critical role in finding the motors for this project - which it turns out is not an easy task.