Hello HeresPaco, welcome to the forum.
Usually used items are around 50% of new, this will vary with age. If a furnace was used just one season, and it looks new, then maybe 75% of new price. It it is several decades old, and looks it, then maybe 30% of new. Don't forget that an item that is 20 years old selling at 30% of todays price may have a dollar value [the number not the buying power] greater than it cost when new.
You may want to consider a stoker boiler as well as a stoker furnace. The hot water from a boiler would be piped to a heat exchanger that would install in your ductwork just like an A/C evaporator does. The advantage of a boiler is that the water stores a fair amount of heat, so there is less lag for increased heat output, the heat ouput temperature is more even, and you can use another heat exhanger to heat your domestic hot water.
We have several forum members that have boilers heating their homes with a heat exhanger [sometimes called an air-handler].
I use a coal boiler in an outside building, pipe this water into the house and shop and use water-to-water heat exchangers to heat the house's hot-water baseboard heat and domestic hot water. In the shop I use a water-to-water heat exchanger to heat the hot water floor heat system.
I think the most difficult thing for you will be to find a good used furnace or boiler. They are not all that common to see for sale used. I'd keep an eye on http://www.craigslist.com
and on eBay. They do ocassionally show up.
Another idea would be to install a Leisure Line Hyfire 2 with the duct top. This stove is a large unit with lots of heat output. But it is not a furnace with a formal cold air in-box and a ductwork heat outlet. This may not work for you at all, but another idea.
Tell us the layout of your house, finished basement, one or two story, well insulated or not, good or poor windows, and the size of your current furnace. This information will help us help you.
I'm sure several forum members will add their recommendations to this thread.
Hope this helps. Greg L