What should we do about asbestos tape on heat ducts

As a certified home inspector I will give you some more specific advice from a Buyer's point of view. If the inspector suspected asbestos, then this warning should be taken seriously. At this point in the house buying negotiations, it would be wise to insist the "Seller" have the material sampled by a licensed tech and tested by a certified lab with a copy of the results given to you before proceeding to a closing. If the tests do come back positive for asbestos, get a quote from a licensed asbestos remediator to remove it, and a quote to replace the ceiling with some safe material such as drywall. I would recommend asking that the total amount of both of these projects be adjusted from the selling price.

Under no circumstances would I advise one of my customers to proceed further into the purchase until this issue is resolved. If positive for asbestos, this could be an expensive fix and definitely not a DIY project under any circumstances.

You need to know how to calculate the required CFM - cubic feet per minute - that your ductwork will allow. On a room per room basis you need to determine how much air flow is needed, how large the vent needs to be, and thus how large of a duct you require. Then you need to determine how large of a trunk you require and at what points you need to reduce it after branching off legs from the trunk.

Finally you need to also know where to position the return air and how large it needs to be, in order to support the output from your HVAC.

I don't know how to calculate those things - I just know that's what you need to know.