The needs for the robot cart change every year (not as much as the robot, but they change). The thing we purchased as a robot cart in our rookie year (
ComboCart) never seems to be quite the right thing. On the other hand, a simple garden cart similar to
this* has been the base of our cart the last four years; we usually spend more time figuring out what the cart needs than in the [re]build. There are so many carts and such available locally between home centers, garden stores, and such that can be tweaked into what we need that it is unlikely we would order something that needs to be shipped.
* - hey, they're available in green now!