Old cement building central heat/air ductwork price

[Jenna Walker]

I am thinking of buying old cement house that is built in 1965.  House does not have central air and heat I’d have to do the duck work.   House does not have crawl space, it has window unit. 

If I redo the whole roof and duct work for central heat/air.  What would be the ball park cost?  House is 1200 sqf. 

Thanks for all the input!