Any of the filtered sands that Home Depot stocks should work for this purpose. You just need to build up an insulating layer to protect the oven and what's below it. 50 lbs. of sand filled mine perfectly. I would recommend placing the lava stones as close to the mouth as possible to leave more room for the wood behind them.
There are some recommended stands listed with the item. I bought this smoker cover and it fits really well: Store SKU #1001534585 It does come with a metal stand for outdoor tabletop use.
That is a difficult question, but it is at least 2.5 feet easy, even after the coals are moved into the corner for cooking. Two pizzas will fit easily.
350 to 425 degrees F is the ideal baking range. The thick walls of this oven will allow it to reach temps in excess of 500 degrees F, but that is not recommended. We recommend the use of a temperature gauge (sold separately) when preparing the fire and cooking to help manage and gauge cooking times.