We have had food carts of all types around our courthouse square during lunch time (except during winter) for quite a few years and they have been very
popular with all the people who work downtown (enough so that some of the downtown restaurants
now have food carts in order to compete). The people can get their food and then sit or stand in the shade of the trees in the square if they want to, so try to pick a place where your customers can eat with some comfort. For our carts, cleanliness is pretty much automatic, since our city/county health department inspects the carts regularly, and will shut them down for major or numerous violations, just like they do the sit-down restaurants and grocery stores.
To me, the bun is just as important as the dog. It should be big
enough and strong
enough to hold all the toppings you might offer without shredding, splitting, or falling apart (and dropping all those "goodies" on your customer's clothes).
You asked what to do to make your cart different. The variety of toppings you offer could make your cart stand out. If I were you I would start off offering all
the locally popular toppings, plus
one or two additional different ones each week, listening carefully to your customer's comments about the additional toppings. Any of the different ones that seem to get favorable comments can then be added permanently to your regular group. From what I have read on the various Roadfood message boards, I can think of well over two dozen different toppings you might be able to offer on a trial basis. Our Boy Scout troop, as a fundraiser, sells hot dogs twice a year at a local celebration and as toppings we offer catsup, yellow mustard, chopped onions, and sweet relish for free, as well as cheese sauce, real chilli without
canned hot dog chilli sauce), and medium salsa, each for an extra charge. (It is surprising how many people will pay extra for the salsa, even though this is not a locally popular topping for hot dogs.)
Good luck, and most importantly, have fun!