Do you have a split system air con unit yet? No? You probably will at some point. Maybe you will use it for heating as well as cooling, as we do.
Ours is a Daikin. Although we had a problem with it in early days, for some reason the circuit board on the exterior part became damp, it is now nine years old and still working very well. It was not especially cheap, I don't think Daikens are. Mother's is only one year old and hers is a cheapie and not a patch on ours.
I am sure they are online calculators to work out the size of the unit you need.
But what you really need is an inverter air con unit. I don't know the tech stuff about them, but what I understand is that instead of the compressor cutting in and out, it speeds up and slows down, making it more energy efficient, and more comfortable and less noisy inside. The maximum exterior decibels the exterior part registers as 65db, relatively quiet.
I really would advise you, don't go for the cheap when you are buying a split system air conditioning unit.