Yes, now is the time to buy accessories.
Much of the dealers cost is getting his installation crew and estimator out to the job. That cost has already been absorbed by the air conditioning system. Also, accessories add a lot of comfort at very little added cost since the major parts: fan system, supply ducts, return ducts registers, etc. are already in place.
There are 4 major accessories you can buy: Humidification, electronic air cleaning, UV light purification and zoning. Of all these, only zoning requires major modification to the equipment. However, zoning does give you the ability to have different temperatures maintained in different areas of the house and to change the temperature in one area of the house without affecting the other areas. However, there is a fairly substantial increase in the first cost.
Humidification is one of the least expensive added costs that also really adds to comfort. It is very inexpensive to run especially with an atomizing type humidifier. If you ever wake up with a dry scratchy throat, have dry skin, or get shocked by static electricity you need a humidifier.
Electronic air cleaning is the best way to clean the air up to 95% efficiency. Do not buy an electrostatic air cleaner. In our test, they really don't take out much dirt, and they have a high-pressure drop, which can damage the furnace or air conditioning unit. If you have allergies, electronic is the only way to go. Most other filters only filter about 30% of contaminates. Also, if you have allergies, you will probably want to add UV lights since they destroy mold, mildew and spores. The product we would recommend is Steril-Aire UVC lights. UV lights in combination with electronic air cleaners have done wonders for allergy sufferers.