I am no expert but in my experience, a furnace will last much more than 10
or 12 years, and that is in a cold climate where it gets lots of use.
At the very least, I would get another heating contractor to come by and see
what he recommends, why and at what price. Even if he charges you a service
call for doing so. A second opinion is worth it for the peace of mind that
you are not being cheated by the first contractor. If the first contractor
objects strenuously to you getting a second opinion, I think that tells you
something.
You might want to post your message in alt.hvac

Good luck!