Repair or replace furnace????

A technician was performing the yearly cleaning on our furnace and said that the walls of the heat exchange are deteriorating--not cracked, but in danger of forming cracks. He advised us to get a whole new furnace. Our current furnace is almost 15 years old, but works fine. We definitely can't afford the new heating and cooling system the company suggested for us, but we don't want to put ourselves at risk for carbon monoxide poisoning, either. (We do have a carbon monoxide detector.) We are planning on getting a second opinion and quote, but in general how do we know who to believe and if we are getting real advice or just a sales pitch?! Thanks in advance for any input!