Unfortunately, the refrigerant recycling laws require expensive service. Whether replacing components or replacing and old unit with an entire new unit, the refrigerant in the machine must be removed without loss to the atmosphere. This makes service expensive, but necessary for the environment.
In terms of air conditioning machinery, a 13-year-old machine is outdated and an energy waster. Repairs will be frequent due to well-worn and used parts. At this point, it is less of a waste to replace the entire unit with a new moderately-efficient unit which is far more efficient than the one you have. You will get totally new parts and can get a service warranty for five years or more.
For a cost of $80 per month, the new 13 or 14 seer unit will run for another three years before you actually pay more than replacing just the air handler for $2300. See: http://www.petro.com/promotions/midatl/MidAtl_Promo2.pdf And it is all one manufacturer. A unit with parts from two manufacturers can not be warranteed and has no efficiency rating at all.