You'll be happy will your choice of Hitzer. I'm a little confused as to why you would save so much money between the two States. It makes me wonder if you are comparing apples to apples. Hitzer makes a model 82 utiltity stove and a model 82 forced air furnace. I would clarify as to which you are buying. For the set up you describe, I would get the Hitzer forced air furnace.
You then duct the supply of our Hitzer furnace into the supply plenum of your existing furnace, using a divided plenum or a back draft damper on you exiting furnace. The Hitzer furnace comes with a forced air circulation blower. You would not want to duct that heated air, under pressure, to your existing furnace return. Your existing furnace blower motor, may not last long, either from the hot air temperature or the added resistance now to the motors, or both. You may use the existing cold air return to supply the return air for your new Hitzer though. This is what they call a parallel system.
Another way is the series configuration. This is where you duct the supply of the existing furnace plenum, to the modified larger opening of your Hitzer return, with the Hitzer's blower fan removed. This configuration only uses the one blower, the blower from your existing furnace, to move the air through you duct. Usually, the existing furnace blower fan CFM needs to be increased to overcome the added resistance though. So, that means buying a larger blower.
Either way, make sure you map out what you are doing before you buy.
Feel free to ask anymore questions.