Do you need a system that is 'cooling only'
or 'reverse cycling'
(cooling and heating)?
This will help you discount a lot of options.
'Cooling only' is cheaper, but only slightly. So, even if you have another form of heating, it's worth
considering a 'reverse cycle' unit as a back-up.
Do you have ceiling insulation in your roof?
Another key consideration. A room with no insulation will require more capacity to air condition than a room
with properly installed insulation.