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Its a pain to order gear reduced low voltage DC motors from Canada but these are some of the best motors I have found on the market. The GM17 is very efficient and works great for solar tractors. We also the GM17 in the LFR classes, however, you may want to consider their other motor types like the GM2. Take a look at the specs for the motor and see what range of voltages it typically operates best at. Keep in mind that you're going to lose a volt or so on your driver transistors so you can never get the full 9V from the battery across the motor. This Gear Motor and Wheel deal package gives the best price.
I like mounting my motors using small rectangular pieces of 1/4 plywood (the track boards and car chassis are made with this) secured on either side of the gear motor with long 2" size #6-32 machine screws.
The Arduino UNO microcontroller PCB is mounted best using size #4-40 machine screws.
Here is the good quality solderless breadboard we use in class. The original car design used a soldered together control board, but I'm going back to solderless to make changes quicker and allow for replacing transistors that burn out. The previous transistors never failed on the soldered board, but knowing my luck on race day, that's when they would blow out. You can get smaller breadboards if you want to save weight, but be careful not to limit yourself.
The design of your optical sensor is critical. Remember you can use more than two sensors if you want and you can look over a wide area, not just inside the line. Here is the photoresistor we typically use. Could there be a potential improvement by selecting a photoresistor with a different resistance range? I don't think so, but you are the designer, not me.
And last but not least, the casters are a big deal for the LFR. Remember what we learned in class: the horse goes in front of the cart. Put the drive motors up front and the swivel caster in the back.
Pay careful attention to the choice on the caster. You may want to burnish the swivel bearing races by hand, they don't work smoothly when brand new. Oops, I just gave away another secret. We'll see who's paying attention and reading this webpage.