Recently, I've been cut off of access to sun and fresh air to my apartment. It is completely sealed now, with no windows or sun to the walls or roof. Ever since, humidity has grown a lot, and mold is spreading everywhere.
The climate where I live is an "always humid subtropical highland climate". That means it rarely gets extremely hot or extremely cold. Because of that, air conditioners are not very popular around here.
My first decision was to look for a dehumidifier, but after some research, and after contacting a manufacturer, I decided it won't solve my problem, because the dehumidifier is made for hot climates only - it has suboptimal performance below 18°C, and will freeze and stop working below 13°C. A climate of 0-10°C is common during winter, when it also happens to be more humid.
So my next option is an air-conditioner, which could give me some fresh air besides dehumidifying. My apartment is ~30m² (~323ft2), and ~3m (10ft) high, and I live with one other person. Some guides written in my language show I'd need 18,000-21,000 BTU, but they seem to consider only hot weathers, which is not my case. A calculator in English considers cold weathers, and it tells me I need only 8,384 BTU, which is less than half.
So, considering I need an air-conditioner mostly for dehumidifying on a cold weather (~5°C/40°F), what BTU power would I need?