As you already have the duct work in place as a result of having forced air heating, it is a simple matter to have a contractor install the air conditioning component. This consists of a condensor that sits outside the home and an evaporator coil that is situated over the supply side of the furnace. The sizing of the air conditioner is crucial because in addition to cooling a home, an air conditioner removes humidity. Lower humidity means that a higher temp will feel comfortable. As you are going to add on to your house, you may want to go with a window unit for now so that you can have the condensor unit sized and zoned with the new addition taken into account. If the addition is going to have it's own HVAC system, then any of the name brand condensors would work. I recently replaced my 40 year old Carrier condensor with a new Carrier unit. Hope this helps.

Thanks Mark208 for your reply.
I was doing some research on the inet after I posted my message and read alot about Lennox air conditioning systems and I was somewhat impressed with what I read. Any comments about Lennox ??