For the OP,
My system is pretty much what you're describing. I'm using a single element (5500) mounted in a keg, and a 10 gallon orange cooler. I just mash with the full volume of water. Check out the "I love no sparge brewing" thread by started by Sacc.
I'm using an Arduino for a controller, as I want to be able to continually add layers of complexity as knowledge and money allows. My plumbing & pumping is all done manually, I'm using the Arduino solely for temp control at the moment. It works as both a PID (to control for mash temp) and a PWM (to control boil duty cycle).
Admittedly, the cost of an Arduino, a power supply, and a display of some sort does approach the cost of an Auber PID. However, it is scalable. You can add more vessels and pump and valve control later without having to purchase more controllers.
FWIW, I agree whole heartedly with crane's last statement above. I don't see the point of going through the considerable trouble of building a control system, if it's not going to control anything for you?