Air handlers and furnaces aren't typically installed side by side in Jacksonville. If you rely on a furnace, you likely don't need to consider an air handler.
Air handlers are typically linked with heat pumps and manage airflow across your house. Some systems also offer secondary heating and cooling components to help the heat pump.
A furnace runs differently by sending hot air into your vents, which move it across your house. As furnaces use combustion chambers to produce heat, they don't need some of the components you'll see in a standard air handler.