Air handlers and furnaces aren't often installed together in Oklahoma City. If you rely on a furnace, you most likely shouldn’t need to consider an air handler.
Air handlers are typically linked with heat pumps and manage airflow throughout your residence. Some models also have secondary heating and cooling parts to assist the heat pump.
A furnace runs in a different manner by blowing hot air into your vents, which circulate it across your house. As furnaces use combustion chambers to create hot air, they don't need some of the parts you'll see in a standard air handler.