If the heat exchanger is truly cracked I would replace the furnace without question. As one poster said CO emissions is never a good thing in a home. One thing to keep in mind is the country has gone to new freon (r- 410 a) your system is 16 years old so I'm assuming it's r-22. Which is being out sourced by r-410a and with that higher costs of the freon and people in the Hvac industry are told it won't be available for purchase much longer. That being said ask the company for a price on a coil with furnace install. This way when you replace the cond unit your ready for the new freon and the furnace doesn't have to be messed with. If feasible though replace the entire system.
I've been installing and maintaining lennox for 10 years and I can tell you it's not junk, anything but. Lennox and it's dealers have some of the strongest customer service and warranties in the business and they make sound and quiet machines.
Any questions feel free to shoot me a pm.