I'd ask for an allowance to replace the entire system, equipment and ductwork at your convience and choice of contractors and then up your loan to cover a full system replacement and get it done right after closing.
With the age and condition of what the pictures showed you'll save on your energy costs, maintenace costs, possibly health costs (if it's sucking in air from the crawlspace) and the difference in you house payments will be minimal. I'm betting you'll save more in energy costs than you'll pay in increased loan payment and that will only get better year by year as energy costs continue to rise.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.