You need an accurate assessment/diagnosis before you can determine a course of action. Just because you smell mold from the ducts doesn't mean it's growing in them. While some duct systems have conditions conducive to mold growth, others are very inhospitable to mold. It depends on many factors like climate, construction technique, etc.
There is more info you are not giving us then you are. Most likely because you don't have the info. The accessibility of the ductwork is more for a pro to determine. The bigger issues are 1) Are you sure its mold? 2) What has caused the mold. I used to do professional remediation of HVAC systems. Even if you clean and get rid of mold in your ducts, if you do not address the conditions that created it or allowed it to grow then you are wasting your time. It will come back. Like others have said, you cannot afford not to call a pro. Long term you are possibly looking at all kinds of health and home damages. Selling the house without a pro signing off will be a legal and liability nightmare for you.
Warning: Just because I am over the head injury doesn't mean I'm normal!