Air handlers and furnaces aren't usually used in the same home in Charleston. If you use a furnace, you most likely don't need to consider an air handler.
Air handlers are often linked with heat pumps and manage airflow throughout your residence. Some systems also have secondary heating and cooling pieces to support the heat pump.
A furnace runs differently by pushing hot air into your vents, which circulate it around your home. Because furnaces have combustion chambers to create hot air, they don't need some of the parts you'll see in a regular air handler.