Two major decisions should guide your purchase of a room air conditioner. Buy a correctly sized unit. Buy an energy-efficient unit. If the room is very sunny, increase capacity by 10%. If the unit is for a kitchen, increase the capacity by 4,000 Btu per hour.
Have your heating system cleaned and tuned by a qualified contractor. A pre-season tune-up and filter change is a good investment. It reduces the chances of breakdowns in the middle of winter, improves safety, and pays for itself through more energy efficient operation.