One things to consider is to figure out if you want just a couple of rooms cool if you want a window unit. If you don't have the space for a window unit. There are portable air conditioners where you can have a vent like a dryer vent installed of if your window in the area that you want to cool and hook up the hose to the vent or the window.

Second you need the area measurements of the rooms you want to cool and this will also help you pick a air conditioner that will work for you and not overwork the unit. These are fairly cheaper ways to air condition the space that you want.

Third is a central house unit that works with your furnace and heat ducts. this one is a little more expensive but is easier to cool the space that you want cool and the noisy part of this one is outside.

So in the end it depend on how much of an area you want to cool and how much you have to spend.