Before you install a new air conditioning system, consider your home's electrical system. It's not uncommon for old houses to have only 110-volt, 60-amp service for the entire home, barely enough power to handle the home's existing complement of lights and appliances. Central air conditioners require a dedicated 230-volt circuit and may require 20 to 50 amps of power, making an electrical service upgrade necessary. Have an electrician size your home's electrical supply up before your HVAC contractor begins work.
Contact your local building inspection department to check for permit and inspection requirements. Be wary if your contractor asks you to get the permit. It could mean the firm is not licensed.