Pneumatic wheels are nice, as they tend to be wide and don't damage turf so much. You can also deflate them for even softer tires, and they hold up well on pavement.
I recommend you think much larger pneumatic wheels, like the size they use on a wheelbarrow. Harbor Freight
sells these for about $6, including the hub with a 3/4" bearing installed. And a whole lot cheaper than AndyMark (not that I don't just love those guys!)
The bolts holding the hub together can be removed, allowing you to bolt on an adapter if you wanted to drive the wheel with a chain or belt for example.