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

However, for cheap programmable robots you can use: the [DFRobot 2WD Mobile Platform]( and interface it with an Arduino Uno. You can also use a [Pololu 3pi Robot]( or even a [PICAXE Microbot]( which is already equiped with a front bumper

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