Here is a handy kit that turns a 21inch mower into a robomower that can be controlled via a remote. The frame is made from tube steel and heavy gauge steel. The robot moves on 13 inch tires driven by wheelchair motors. There are also 6 inch casters in the front for easy maneuverability.
The kit weighs about 140lbs in total. It comes with motor controllers, batteries, pneumatic wheels, and everything you need to get started.