I just got ~$2000 to buy arduino components, boards and misc for middle school students. I want to purchase a 4 wheel and a tracked robot base. I found this: https://secure.robotshop.com/en/dfrobot-4wd-arduino-platform-encoders.html Do I need anything to help interface with the arduino? Any suggestions?
You will need a motor controller to control the motors.
I would contact Sparkfun if you are in the US. They like to support this kind of thing. I know they at least had a robot thing but I can't find it offhand.
If you want to stretch your money, you could get four of the below servos and modify for continuous rotation, hot glue them to a piece of thin piece of 5.2mm plywood, attach large plastic jar lids to the servo horns for wheels, and have a 4X4 bot platform. bottom is a project using the same approach.
Personally I like this better from Roboshop: DFRobotShop Rover V2 - Arduino Compatible Tracked Robot (Basic Kit) - RobotShop. I'm not sure its right for middle schoolers, might be to sophisticated.
However, most middle school programs use Lego Mindstorms, Paralax Boebot or VeX. Legos are challenging enough for middle schoolers and you can use different software should they prove more advanced. The advantage of the kits is they get something going. Building from scratch, like buying a chasis w/o motors, can be frustrating for noobs.
And 2k is not much. So its good to put the kids in groups and let them play.