Hi, a few pointers based on what I have messed with:
Chassis/Motors: You mentioned Dagu products, from different suppliers. Dagu information and the manuals for most products are here:
https://sites.google.com/site/daguproducts/ (Thanks to Russell Cameron, who designed many of these, and the great Magician controller boards)
Take a look at: http://goo.gl/uBg9K (Disclaimer below) for some Robot stuff I have tried out..
The Robot 7-way Multi Tracking Sensor works well and is versatible for line following. The Ultrasonic sensors are best for obstacle avoidance. Putting one on a "Sensor Pan-Tilt" head allows the robot to look around while travelling ahead.
Sometimes Servo-based motors/wheels make it easy to get started with a robot. Simple 3-wire connection, built-in H-Bridge. But not highly powerful. Fine for travelling on a typical floor/carpet.
If you're going to use DC motors, the Micro magician controller has good FET-based H-bridges built in, plus an accelerometer to know when you hit something or fell off a cliff... And separate Servo power connections. Russell has thought a LOT about all this.
Keep narrowing in on what you really want/need...
Other: If you're doing a course, check out the ArduinoInfo.Info WIKI for class materials, like
http://arduino-info.wikispaces.com/Arduino-What-IS-it and http://arduino-info.wikispaces.com/YourDuinoStarter
DISCLAIMER: Mentioned stuff from my own shop...