It is worth buying an inverter* type rather than the slightly cheaper non-inverter versions. There isn't a big price difference now.
I have 9,000 Btu versions in each bedroom, which are about 4m x 4m x 2.7m each, and a 24,000 Btu version in the living room, which is 7m x 4m x 2.7m. I could probably have got away with 18,000 Btu in the living room but I had previously had a 12,000 Btu version and that was not sufficient so I wanted to be sure. You can get an idea of prices from e-shop
. Obviously fitting would be extra but not that expensive. I think makes like Samsung, Mitsubishi, Toshiba have a good reputation.
In order to fit the ssytem it is necessary to drill a relatively large hole through the wall in order to pass the pipes and cables. This means that you usually cannot fit them immediately above a window since there is often a reinforced concrete lintel there.
* I think people often misunderstand terminology when it comes to air conditioning units. A lot of people seem to think that "inverter" means that it can both heat and cool. That is not the case. I think virtually all air conditioners can both heat and cool.
A standard air conditioner has a compressor that runs at a fixed speed. It controls the temperature by turning the compressor on and off for periods of time. By altering the ratio of the on/off time it can vary the amount of cooling/heating. This is the same way that a central heating boiler works. An inverter type on the other hand adjusts the speed of the compressor to produce more or less cooling/heating. Generally inverter types are more efficient using less energy and therefore cost less to run than conventional types. They tend to achieve the desired temperature more quickly and they maintain that temperature more closely than a conventional type. They cost a bit more because they have more complicated electronics for the DC/AC inverter to drive the compressor while conventional types just have a simpler on/off controller. When comparing different makes look at the efficiency rating.
I'm afraid it is often a balancing act between higher capital outlay/lower running costs or vice-versa together with reliability and longevity. I initially went the low capital outlay/higher running cost route as I didn't know how much I would use them. When I changed the living room unit I went the opposite way with higher capital cost/lower running cost because I found that I use the living room one quite often in the summer but use the bedroom ones much less.