Well I just installed the same furnace a week ago in my house, so I guess I can comment on this. I did some research online concerning the Goodman furnaces. You will read some bad stuff and some good stuff about them. They are positioned in the market as a lower-end furnace. I do not think this makes them a bad furnace, most all furnaces use the same guts, controls, blowers, gas valves, etc just in a different steel box & heat exchanger. Their warranty is among the best in the business. The big difference I found with Goodman is that their distribution network is not as well regulated as most of the other brands, so pretty much anyone can go buy a Goodman furnace and install it. A lot of fly-by-night hackjob installers install goodman furnaces and the installation problems give the furnaces a bad name. In any case you should make sure that your contractor is qualified to do the work and has the necessary licenses & insurance... be doubly sure if he's installing a Goodman or Amana(made by goodman) sine their distribution is "looser."
I actually installed mine myself. It's not rocket science if you're comfortable with plumbing, electrical, and sheet metal work. FYI I paid $850 for the down-draft version of the model you listed, including the propane conversion kit. And I bought it through a dealer who still made some profit on the sale. So that gives you an idea of the actual cost of the furnace versus what you're being charged in labor/profit.
One thing I didn't like about the Goodman multi-speed furnaces is while they use a dual-stage gas valve, the controller does not allow for true 2-stage heating with a 2-stage thermostat. The control works only with a normal 1-stage heat thermostat and uses internal logic to decide the stage. Actually the factory default is just single stage operation, and there are dip switches you can set to select 2-stage operation and the time delay to switch to high heat. What it does is start up on low, then after 5 minutes or a variable delay it switches over to high if the t-stat is still calling for heat. A true 2-stage thermostat would run the furnace on low unless it high heat was absolutely necessary (very cold day or just turned up thermostat). This is probably not a huge deal but consider it when comparision shopping. The Goodman variable speed furnaces do offer true 2-stage heating at about a $250 premium.
What all is included in the quote? It seems high to me for just a remove/replace. If he's got to do ductwork changes or add a long PVC vent it may not be a bad price. One thing though, did the contractor do a heat load analysis on your house or did he just guess at the size of the furnace? A good installer will either do a "manual J" calculation manually or use a piece of software to do it. You can actually buy a limited use license for the software for $50 to do the sizing yourself, I used it before ordering my new furnace and it's pretty slick, very easy to use.
www.hvaccomputer.com is the website for the software.
Let me know if you have any other questions.