In SD you can get a Limited Item Food license that allows you to sell only a select few items, usually ice cream, popcorn, hotdogs, coffee, pop and candy, and items like that. For full service all you need is a 3 compartment sink- any size will do. I have seen some realy small ones like you see in a small camper. I fresh water supply- can be as simple as a water tank or a garden hose. And a place for the waste water to go- another tank.
As far as a commercial refrigerator, all you need is an accurate thermometer in your fridge or freezer- you can pick them up for as little as $3. The ones with the temp reading on the outside are nice but not neccessary.
Usually you need a handwash sink but a small wash pan with a water jug over it will meet code.
I don't recomend going this way. If you are going to do it , do it right. Or as best you can with what you can afford. Food saftey is the number one priority when selling to the public.
Some people are like Slinkys, they are good for nothing but can still bring a smile to your face when you push them down the stairs.
Down to one lonely ol' Bandera.