Depends on what you're planning to do with your rig and where you plan to do it.
If you're looking to serve food as a biz you'll need hot & cold water, a 3 bay S/S sink for wash, a separate sink for hand washing, a potable water approved tank, a grey water holding tank and more.
If you're just looking to rinse your hands & utensils a simple tank with a pump will do, a small water heater would be nice.
If your using your rig at parks & fairgrounds, most all of them will not allow you to let a sink drain on the ground so a complete drain system & holding tank will be mandatory.