Air conditioner

The attic is going to be very hot in the summer time and it would help if you put an exaust fan in the attic to lower the temp in the attic. I don't know what you insulation situation is but that is one of the cheepest things you can do is to put plenty of insulation where ever you can.

Now about the adding Airconditioning to the existing furnace. All of Carrier uurnaces made in the last 15 or so years will be wired for AC and have the needed relays. You would have to add an evaporator coil above the furnace, if it is an upflow unit and under the furnace if it is a down flow. You would have to run two freon lines to the area that you are going to put a condensing unit. Also you would have to run high voltage to the outside unit along with some low foltage wires to the outsice unit and the furnace aand the thermostat.. One other thing which is probably already done is that you duct would have to be wraped with vapor barrier insulation.