You can find some of the hardware used with Pi Robot in this blog post. However, the Serializer was discontinued a long time ago so now I use an Arduino Mega and Pololu motor controller instead of the Serializer and Arbotix.
My personal recommendation is to buy a Kobuki (Turtlebot 2)
instead of building your own base. It is well made with full ROS
support right out of the box. The main limitation is that it is
strictly an indoor robot (small wheels and very little ground