Hey Money - Couple things, and it may not be an issue there in TX, but would be here in CA. 1 - Here, you cannot cook in another vehicle unless the product is transported to the serving vehicle in NSF containers. 2 - Here we need a seperate order and pick up window, as the HD does not want money and food passing thru the same window. 3 - Center aisle needs to be at least 34 inches.
We do bottled drinks due to logistics of fountain sodas (Ice, water, bag in box storage) and we can replenish anywhere. Think about plenty of overhead shelving for bread, food containers, etc. Onan generator if possible, propane powered if possible (Refrigeration depends on a good generator).
We bought existing trucks, (both 98's) which are gasoline powered (Easier to find mechanics, based on the odd hours needed for repairs). We are fast, and serve quality, and can do about 80 tickets an hour. We run the truck with a max of 3 workers.
In our 3rd year and still going strong, but there are always issues. PM me if you want to talk - Best of luck.