In that case negotiate a decrease in price based on replacing the furnace, then hire YOUR OWN quality contractor that will install good equipment for you. For example get three QUALITY contractors to bid replacement, then based on the estimates you receive take that off the asking price of the house. You may want to consider a high efficiency heat pump or a 90% furnace. It will be worth the extra money.

DO NOT force the original owners to REPLACE the furnace themselves...they will put in the cheapest junk they can find...who wants to spend money on a house they are dumping?

To sum it up, get the owners to subtract a reasonable amount from their asking price and then hire a good company that will do a good job installing good equipment and high efficiency is worth considering...