If asbestos is in the ducting or the duct work or in the ventilation system period this is not good and I would not buy the home or even breath inside the structure.
Now, I would get a clarification on the findings, if the findings actually stated asbestos "IN" the ducting or "around" the ducting. In the ducting is not good as I previously stated because that means the particles are being blown into the inside of the home. If the asbestos is around the ducting, then this can be removed and proper insulation installed.
Get a clarification and I would recommend not to persue this with further serious intentions until more information is found about this situation.
Make sure you totally understand what is going on before you sign anything and if you do not understand something stated ask more questions or just get a second opinion.
Asbestos will kill and it will take a long time, but it is an accumulative agent that is not flushed out the body or lungs.