If it is previously used,,, it is not worth a penny in my book except the scrap metal worth..
If it is NOS(new old stock),,, there are much better models on the market today and would not be worth as much as the models of today..
Second, are you sure you need 100,000btu,, did you have a contractor do a proper load calculation... Make sure you find a contractor willing to install a customer supplied furnace before you buy one.. Most quality hvac contractors will not install a customer supplied unit and if they would you would not be saving any money and may have problems warranting parts(basically no parts warranty)..
As stated many times before,, get a great contractor and go with the best system that you can afford from that contractor... Proper installation is the most important factor with any brand..