We bought the house 2 years ago, built in 1991. Heat-a/c is original, had a spring tune-up done and the tech said coils were corroded, detected freon leaks, and the compressor is weak. It makes sense, the a/c runs constantly as soon as the temp gets to 80+, but it can't cool the house below 74. Since we're getting the a/c replaced and the furnace is part of the unit, we're doing a full replace. Basically, we're getting the house ready to retire in.
They do have a serviceable life and it sounds as if you are at the end of it.
What type of system are you putting in its place? Have you considered GeoThermal? The application value of that depends on thermal conductivity of the soil and ground temperatures.
The overall price of a GeoThermal system will far exceed the cost of a high efficiency AC and Furnace, however, you will get a tax credit and it will be paying the added investment back immediately.