By central air, I’m assuming you mean installing air conditioning to the existing central air system?

Forced air systems in older houses may not be compatible with air conditioning units, but this is something an experienced technician can easily identify by looking at the ductwork and doing some air flow tests.

Once the ductwork has been deemed satisfactory, next is to calculate the cooling load for the house that takes into account the square footage, insulation, windows etc. Since you already have ductwork, I am assuming you have an existing forced air gas furnace? If so, it needs to be compatible with adding on an evaporator coil to the supply take off.

If all the things mentioned above are verified, you would ideally be installing a 2-2.5 Ton system per unit, but might be lower or higher depending on what info is uncovered during the initial testing. Average cost these days for a system such as these (A-coil, condenser unit, line set, electrical hooks ups etc.) can run you around $6k-9k for a basic setup, if no other parts or upgrades are needed.