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