Mold in the house can be controlled only by regulating moisture level. You should have a dehumidifier in the house which will purge excess moisture present indoors. If your house is well insulated and airtight, purchase a mechanical ventilator that will allow entry of fresh air inside. It will replace the stale air indoors. Lastly buy a hygrometer that will show the humidity level indoors. Whenever the level goes up, you can take the required measures to control the moisture level. Remember no moisture, no molds.
Molds are often found in AC ducts. Leaky ducts allow moisture to trickle inside. Moreover the air that passes through the duct brings lots of dirt with it. As a result gunk builds up in your ducts. The matted layer of dirt and moisture provides fertile ground to the molds to live and grow.
You should always avoid buying an oversized unit. An oversized unit will cycle on and off quickly. It shuts off as soon as the air is cooled but before it is dried. This insufficient drying of room air leads to build up of moisture in the house.
If you suspect mold growth in your ducts, call a professional duct cleaning service ASAP. They will clean up your ducts and spray biocides to terminate its growth. Always ensure that an EPA approved biocide is used. The chemical should be administered according to the instruction on the label. Mold growth in the duct can be detected by bizarre discoloration at the vents. You will also get a musty smell from your ducts. However some people tend to think that any black patch in the duct is a sign of mold. No it is not so. If you see dark patches, take part of it on a sticky tape and send it to nearby microbiological laboratory to confirm whether it is mold.