First of all see energy rating label on the system. Generally, this rating label has stars between 1 to 6. The output of these systems varies by the rating labels. These labels identify the energy efficiency of the systems. The more the stars the more system is efficient.
You should also look for programmable thermostats. They are the temperature controllers. You can turn the system on or off according to the need and requirement. Some systems have internal thermostats and some have separate thermostats, so if you are buying a system with outer thermostat, then place it in dry place and not near the grille.
Ducted system should be insulated properly; a poorly insulated system will not give desired results. So, look if the joints and seals are properly sealed or not. You can hire insulators to insulate systems for your home.
So, this way you can buy a proper air conditioner for your home to enjoy warm environment inside your home in winters and cool in summers.