Air conditioning new building experiences?

It will hardly have any effect since cold air falls down as already mentioned. I would go with the option of 4 indoor units. Retrofitting will be much more expensive. Especially since you have no ventilation. Why do people still build without controlled residential ventilation? The houses are very airtight. If you don't ventilate sufficiently, you will get mold problems. With the air conditioner, you can at least dehumidify...

Because a ventilation system was supposed to cost 16,000€ just like that and other things are more important to us for that money. Unfortunately, money is finite. In addition, sufficient ventilation is ensured in the warm season, windows are practically always open. Only in the cold season will we have to ventilate consciously.

I am also looking for an air conditioning system. Old building, 1970, no central ventilation, facade will be insulated (therefore now is the optimal time for a core drilling). Primary need - my study - 10 m2. Optional/later bedroom (both rooms next to each other on the ground floor). How important is fresh air supply? The HVAC technician says just ventilate (warm air comes in) and then it is cooled. In the study I ventilate very often in autumn/winter/spring - I have a CO2 meter. When you are alone in such a room, you also have to heat the PC/screen additionally. I have roughly looked at Daikin Sarara Ururu (no longer offered by the company because too expensive), Mitsubishi M-Series, Panasonic and Samsung. An HVAC technician wants to offer TCL from China. Are there any experiences with the manufacturers? Thank you very much

At the moment, many things are probably hard to get. We had the Mitsubishi devices in the old house, including in the office, and were satisfied. This time we are getting Remko, about which I cannot make any statement yet.

Must also choose according to volume. Samsung and Daikin claim 20 db inside, 40 outside, hard for me to believe. How can you "test" such an air conditioner?

I only use Daikin now. You can't go wrong with it. They are also whisper-quiet at the appropriate setting.