It's pretty much the simplest thing that could possibly work. I used one of my old servo brekout boards as a base (as I have lots of them). Two servos with stripped gears had those gears removed and were converted to continuous rotation. A Pro Mini acts as the brain. Two SMD switches are attached to the whiskers, which should be enough for basic obstacle avoidance.
As small caster ball acts as the third wheel, and a small LiPo battery provides the power.