You might consider googling "BEAM robotics"; this stuff is fairly simple and generally doesn't use any micro-controllers. Here's a few links to get you going:
http://costaricabeam.solarbotics.net...Vores_Main.htm
This has several solar and battery powered light seeking robots.
http://costaricabeam.solarbotics.net...thers_Main.htm
The "edgebot" circuits here are similar to what you're looking for, i believe.
http://members.tripod.com/robomaniac_2001/id120.htm
This is a tutorial to build a simple robot using only a few switches that is capable of following a wall, etc.
Regarding the use of PWM signals, its really not necessary. First, you can obviously just remove the controller from a servo and use outputs from some circuits to directly drive the motor. If you need greater current capacity, you can use a chip such as the 74AC245 ("Octal Tri-state Bus Transceiver") as a simple motor driver. You can use one chip to drive 8 outputs, or you can wire sets of inputs and outputs together to deliver greater current. For example, one chip could be used to drive two motors wherein two sets of four inputs were wired together and their outputs were also wired together. If you need still more current capacity, you can stack chips on top of each other (be sure they have the same orientation), which essentially wires all the corresponding inputs and outputs from the two chips in parallel.
This may sound confusing, but if you poke around and do some reading it will all make sense soon enough.
Hopefully this helps. Enjoy your project!