First check out if your apartment or house has got pre installation for air condition. It is so much easier if you can take use of a pre installation. You will then only need one indoor unit and one out door unit. The cold or hot air will be distributed through a sprinkle system throughout the house. If you would like to individually set the temperature in the different rooms that is more complicated and will require adjustments to the pre installation. When choosing the units you can choose between the inverter type which is a bit more expensive but will save you aprox. 30% on your electricity bill or the standard which is cheaper to buy but will give you higher electricity bill.
The size of the rooms are very important when considering the type of machine. If you talk to a air condition specialist he will take in consideration factors like amount of windows, with or without awnings and the orientation of your house, how many live in the house and the amount of sun hours your apartment are exposed to when recommending you a machine. You can also buy the units yourself at places like the Carrefour in Benidorm or the Alcampo in Alicante.
When everything is done it is important that you clean the filters regularly and book an inspection check up at least once a year by the air condition installer. This will help your air condition system to last for years.