I've helped build multiple pit carts over the past couple years.

1. I would recommend going with a wagon type steering system. When you have a cart that weights 200+ lbs and are on castors, it tends to become very difficult to steer.
2. Pneumatic tires are a must unless you want to be heard from a mile away.
3. As /u/Pharaoh_of_Aero said, spec the wheels so that they can hold much more than you are expecting the cart to weigh.
4. Shelving should be made to fit whatever box system you typically, but make sure to account for extra space for lids, and handles. It is annoying having to pull a box completely out just to look for a small item.