I believe that airbnb flat is the best option for your situation.
The price of equipping this space as an office, shop, gallery or a flat shouldn't vary too much between the options, since all of them require making of completely new water, canalization and electrical installations. What makes these various options different from a standpoint of investment is furniture. In this sense, a flat is probably the most flexible option, since it allows you also to buy some charming cheap stuff on the local flee market.
Looked from the usage point of view, the flat is also the best, since you can always keep it occupied at this location in the city, and the amount of work you need to put in is not that big. Compared to the revenue you can get by renting the space to be a office or a bar airbnb is by far the best solution. (unless you want to run the bar by yourself, but that again is a larger investment and incomparably more work than airbnb).
The main argument against the flat is the lack of natural light.
I don't think that the lack of sunlight is a huge problem, since the people renting the flat are tourists, who spend there a short period of the time, and are in the flat early in the morning and in the evening. They will spend most of their time in the living / dinning area, where they do have enough natural light.
So here it is. I propose a loft, where you can provide accommodation for 2-4 people. It is designed as a hybrid of old and new styles, and it allows you to easily furbish it according to your budget. I wanted to make it grungy, and yet light and welcoming. The proposed furniture scheme is there in the first place to describe colors and sizes of individual pieces. Most of this stuff you can replace with something else, as long as the size, texture and color match.
