5 flat storefronts are enough. As you said, if the villages and towns do it, the decoration will generally be fine. Make a paint, then make a few cabinets and put two beds on it.
In fact, this thing is unclear. You'd better visit the market yourself, or visit the city-level stores to gain more experience.
as for how much money to invest, it also depends on the quantity of goods distributed in your shop, the decoration cost, the funds of noodle shop, and so on. ...