Replya new furnace is better equipped to handle the needs of the air conditioning system. A furnace that is designed to operate with the AC system you are installing is even better equipped. At 20 years old, your furnace is living on borrowed time anyway. The installers will need to disconnect the ductwork to install the coil, do you really want to pay them again to disconnect the ducts in a year or two to replace the furnace? It is always cheaper to replace both at the same time as far as installation costs go. If it isn't, your contractor is fooking you. It may be more expensive than just replacing the AC, but it definitely holds the best value.
As for the air cleaner, I believe the accuclean/cleaneffects system has components that must be washed by the homowner. It's a good system that works very well, but YOU will be resposible for maintaining it. If you are thorough and always do all the maintenance around the house on time, then it shouldn't be a problem. Most electronic air cleaners that i come across are dirty and don't get cleaned by the homeowner like they should. if this sounds like something you may not keep up with, then I would say go with a replacable media filter, like the Aprilaire 2200. The 2200 is a very good air cleaner, if you want a step above you can go with the 5000, but unless you have someone in the house with severe allergies or asthma the 2200 is usually more than adequate.