The most important criteria to bear in mind at the time of purchasing an air conditioner:
The type of air conditioner, whether fixed or mobile, one or several units.
Whether it is also capable of heating by using a heat pump, cooling power, expressed in BTU/h or Watt.
You may also consider: energy savings that it may provide; noise levels - which must be low; dehumidification capacity; the presence of additional functions such as a timer for programming, "sleep" to avoid temperatures dropping too low at night, air filtering systems, etc.
Finally, you must also take into account installation costs for fixed air conditioners and periodic maintenance operations.