I requested a pre-winter check-up on my heating unit from a local service company. They informed me that they found a crack in the heat exchanger at the roll joint. The service man made the diagnosis on visual inspection only (I couldn't find the crack). He said he couldn't turn on the unit based on the visual. He recommended that I get a new unit since they are much more efficient and it would only cost me 300 - 500 hundred dollars more. I have a 1990 TRANE Attic Central AC/Gas heater.
1) Am I getting burned? How do I verify there truly is a crack 2) Is it true that to repair the crack, a whole new heat exchanger would need to be installed and that it costs almost as much as a new Dukane unit. ($1500 vs $1900) 3) If I have to bite the bullet, is a Dukane a good unit?

Thanks for your help.