I used to build benches like this. Fixed casters on one end, directly mounted to the corner posts. On the other end, posts that extend to the floor. A cross-member with swivel casters (to allow turning the bench when rolling) mounted on a piece connected to a pedal. When I step on the pedal, it shifts a cam, forcing the casters down, lifting the bench legs about 1/2" off the floor, and allowing it to be moved.
When the bench is positioned, kick the pedal up, the legs return to the floor, and it is completely stable.
I built about 10 of these over the course of a year.
The nicest one I built had a 6 ft x 3 ft, 1/2" steel plate top, with a large wilton vice. It was completely stable, but easy to move over a concrete floor.