Houston HVAC options?

Our 1900 square foot house was built in 2007 by Ryland Homes. The house came equipped with a Carrier three ton air conditioner and furnace. Two days ago the temperature in the house was 82 degrees so we called the company that installed the system for a $199 maintenance call. The technician said that the system is probably leaking and was low on Freon. He said that the charge to find the leak would be $250 and repair would be $3,000. He also said that the repair might not work for long due to the age of the system, it could start leaking again from somewhere else.

He suggested that we consider getting a new system. The first quote was for a $28,000 system discounted to $22,000 that included everything down to ducts. Today’s quote was for $10,800 that just includes the air conditioner, evaporator coils, and furnace. The quotes came with a low monthly payment using 12.9% financing for 120 months which is ten years.

My wife and I are living on a fixed income and not working. This is an expensive thing that we have to address. Life in Houston without air conditioning is not an option. Do you have positive experience with a company that I can call for a second opinion? I plan on getting at least four quotes before making a decision.