I had hoped to start construction on an outdoor masonry wood burning water heating stove this fall, but it hasn't happened yet. I have the fire brick, but I need to build the foundation for the stove and the adjacent cistern. I'm out of cash, and the first bucks will go to the parts to electrify the garage. So I don't see getting started on the cistern and stove until next spring. Oh, well.
The plan is to heat the water and pipe it into the garage and use some old car radiators and fans to circulate the heat. Ken Kern wrote a great book on building masonry stoves. It is my primary source of design information. They are not all that complex and don't require special skills. The materials are cheap, unless you have to buy new fire brick.
The models I looked at on the internet were all expensive. One common design puts a water tank on top of the fire box. I don't think this is a great idea based on what I learned from Kern's book. Most also have stainless steel fireboxes. According to Kern, steel boxes don't work as well as firebrick, because they don't allow the chamber to get to the optimum temperature. Setting the water on top, or against the fire box makes it even worse. You get the most BTU's out of wood by flash burning at very high temperature. You cannot reach these temperatures with slow burn, metal box and adjacent water.
The way Kern suggests heating the water is to route the flue like a russian fireplace and capture the heat from around the flue.
The most expensive part is the fire brick. New cost is in excess of $3 each. I searched and found some for 15 cents apiece. The stove and water heating features do complicate the construction and increase the price. A simple outdoor woodburning heater would be very cheap and easy to build if you can find the used fire brick.