The gas valve ordinarily would not affect the richness of the mixture. That is determined by relative sizes of the orifice and venturi and the setting of the air shutter.
Should you need to replace the furnace you should be able to obtain one for much less than $2000.
Suburban SF Furnace - Heater at the Suburban SF Furnace Catalog
Depending on the model of your existing furnace it may be necessary for the installer to build a new base for it and make other adjustments. Even so, a technician familiar with Airstreams should be able to do it in no more than 4 hours.
I would not install a used valve. If the gas valve has indeed failed, I would imagine that new gas valves should in fact still be available. Parts diagrams, lists, and prices for most furnaces of recent manufacture are available here:
Suburban Furnace Parts for Sale - Atwood, Duo Therm