I have built a SHR robot using an arduino and a L298N. Everything works fine although the motor driver is junk. However, I want to build another. One that is really small, and cheap to build. I have not bought any parts so everything I am saying is just thoughts in my head right now.
Remember that all these parts were (mentally) chosen due to their small size and cost.
Arduino Nano - $8 eBay (Has PWM pins which would be used for servo, etc.)