So if I'm reading this right, the ducts are running
under a concrete slab foundation?
If you need help deciding to walk away, picture half a dozen sweaty, ugly guys like me jackhammering and digging trenches through every room in your house. Picture yourself writing several ridiculously large checks to destroy the house and then put it back together again without getting anything special or rewarding from it, other than a properly working HVAC system.
You won't be able to live in the house while they are doing it either, so picture paying 100 bucks a night for a month to live in a hotel or picture your life in a tent in your back yard.
And if the ducts are collapsing, imagine what ELSE might have been shoddily done when that house was built that you don't know about yet!
Now picture buying an equivalent home for the same price
without that problem. Easy solution? Walk Away!