Midium size mean? How big of the overall dimension?

Not sure if 3x4m then go for 1hp for bedroom and living hall 1.5hp or 2hp should be enough.

Brand can decide York Cooling King, if you wanna go for budget and come with higher BTU also

Daikin my neighbor using it, less than 2 yrs already feels like he using 6 yrs condition, for outdoor too noisy already

For me, I don't fancy those thing. One item more, may one one more headache in future.
Also, too clean condition may not good for body self immune system sometimes.

Bacteria and virus exist naturally in the environment.
My view, an ordinary clean condition is good enough already, or in other word ensure, filter being cleaned periodically, cooling fin being cleaned properly, drainage system clear of dirt (prevent fungus growth).

Anti-bacterial soap
Anti-bacterial floor cleaner
Anti-bacteria washing liquid
Anti-bacteria air-filter
Anti-bacteria lotion
Anti-bacteria humidifier.
......, you name it, not enough anti-bacteria? 

Also too much anti-bacteria/virus substance used could lead to stronger bacteria/virus growth in the future, as they do mutate and change according to environment.

In some sense yes.
Those plasma filter you need to change them every year. Not just install and can use till failed.

So far hitachi DC inverter will turn on yellow light to indicated remove filter and wash.

Sharp should be nice right also got built in Plasmacuster.