The Zumo 32U4 robot from Pololu is built around an Arduino-compatible ATmega32U4 MCU. Motor drivers, an LCD, quadrature encoders, line sensors, side and front proximity sensors, and a full IMU are all part of the kit. In fact, the kit includes all the parts you need to build the bot, minus a pair of micro metal gearmotors and four AA batteries. The bot could be used for many basic robotics projects and is small enough to qualify for Mini Sumo.
You can buy the bot on Pololu’s Website for $99.00. Some assembling, including soldering, required.