You haven't define what's the limit of your budget.

However, for cheap programmable robots you can use: the [DFRobot 2WD Mobile Platform](https://www.dfrobot.com/product-65.html) and interface it with an Arduino Uno. You can also use a [Pololu 3pi Robot](https://www.pololu.com/product/975) or even a [PICAXE Microbot](http://www.picaxe.com/Hardware/Robot-Kits/PICAXE-20X2-Microbot/) which is already equiped with a front bumper

Look a this for more kits: [47 Programmable Robotic Kits][1]


  [1]: http://www.intorobotics.com/47-programmable-robotic-kits/