After building this
http://www.chiefdelphi.com/media/img...51686d05_l.jpg
I can tell you that you need more than one battery if you plan to have this thing run actively for more than an hour. Pushing 200 lbs around isn't much fun and an extra 40 lbs of battery would give you a safety net.
Ours is a six wheel drop center design using wheelchair tires mounted onto wheels from Harbor Freight. A little welding and a sprocket mounts nicely to them, and they only cost $4 each. Upgrading bearings is a must, but the benefit is that they are steel and we don't have to worry about cracks or issues with rough operation that will happen off of a traditional playing field. Ours is only two wheel drive and we have issues with spinning tires in grass. Four or Six wheel drive would be nice even if it would hurt battery life a bit more.