The duct work is a thin guage metal (30/26ga.)and has
probably rusted and collapsed causing sand/gravel to fill
that void.
Putting system in attic is good option.I would recommend
90% gas unit, direct vent. As far as price, ask several
contractors for cost.
If flue pipe is disconnected in attic must be corrected
serious co issue.

Hopefully the offer was contingent upon the results of the inspection. It's a buyers market and the cost of repairs should be negotiated into the offer. Get several estimates that list out exactly what will be installed for equipment, supply and return registers and grilles (Count), general ductwork layout, controls and any electrical work. If they need to open up the walls or ceilings ask about that too. Ask about their License, bonding/Insurance and references. If it is not in writing, it is not in the contract.

Inspecting the ductwork interior with a camera should be possible. I saw some slab houses years ago where the buried sheet-metal ductwork was encased in concrete before the slab was poured. Not necessarily energy efficient, but durable.

Thank you for you input about the heat and air system. It was especially interesting to here about the co2 problem with a disconnected flue pipe. I hate to think what would have happened ig that had not been found and we had moved in there with our two young children!

The problem is CO, carbon monoxide, results from incomplete combustion of carbon. CO2 is the desirable byproduct of complete combustion. It is the gas that gives Pepsi it's fizz.