I know this does not directly answer your question, but...
When I was faced with that same issue a year ago, I did not even hesitate to purchase a new NT30 at $420. The thought of putting significant amounts of money in a furnace 17 years old, and more importantly, a heat exchanger 17 years old, did not appeal to me. And I am a confirmed cheapskate.
However, had I been thinking I would have gone over to the local RV salvage and got a used one for $250. They get units in all the time that are one or two years old. It is quite possible to find an unused used furnace.