First, make sure the unit is properly sized. Your dealer will provide a load calculation for your home. Also ask the dealer to provide an energy analysis to determine operating cost. Next, consider any comfort issues in the home. Some products can reduce air stratification and uneven temperatures from room to room. If you have allergies, an indoor unit with an ECM motor will allow you to circulate the air in your home continuously while filtering the air for about the same cost as operating a standard light bulb. Finally, know your budget parameters and the efficiency of the system being proposed. Does the system offer a payback? In other words, will the monthly savings over time offset the cost of the new unit or efficiency option being considered?