Sounds like your first hvac company was ripping you off. Ask your new company to go to your coil and see if there is a leak. If there isn't the last company was just lining their pockets. If you have had ducts unhooked for a long time I would recommend duct cleaning, trust me I'm not a fan of duct cleaning myself. I have done hvac for 20 yrs. in GA. When I just bought a home built in 1996, we had all the ducts replaced immediately.
Your contractor will do a load calculation to determine the proper central air conditioning unit for your home. This calculation accounts for the climate, size, shape and orientation of your home, as well as its square footage. A professional will also look at the insulation, windows, walls, floors and other materials that compose your home. He/she will then examine any leaks, seals and existing ducts or vents.
They were quick to come out and diagnose the problem with my furnace, but I thought the estimate of $549 a bit high to replace the circuit board. So called around and found Leo's Appliance in Concord that sells appliance parts and the circuit board was only $90.49 - that is the retail cost to me, so I imagine the wholesale cost is a lot less. So BEWARE of this company since they wanted $500!!! to replace the circuit board which is located right in the front of the furnace - extremely easy to access. I hate it that you cannot trust repair companies.
Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. This is because much of the dirt in air ducts adheres to duct surfaces and does not necessarily enter the living space. It is important to keep in mind that dirty air ducts are only one of many possible sources of particles that are present in homes. Pollutants that enter the home both from outdoors and indoor activities such as cooking, cleaning, smoking, or just moving around can cause greater exposure to contaminants than dirty air ducts. Moreover, there is no evidence that a light amount of household dust or other particulate matter in air ducts poses any risk to your health.