I have used a Lange 3.5Kw stove for over 40 years and love it and this is now the third house that it has been in. It was the intention to use it occasionally to back up our central heating but is generally used continually. I do get through a huge amount of wood but really enjoy collecting it. I built a summerhouse three years ago and managed to find another similar small Lange stove on ebay for £42. It is a perfect size for the room which has four large armchairs and it is now the favourite room "in the house" and my wife seems to be constantly using it for entertaining. The fan heater option is good and takes up less space, but for the right person the stove route is best in my opinion. From my experience I would add;
Use a double lined insulated flue to avoid condensation with as long a run inside the building as possible.
If the stove is too big it will not be running hot enough to keep tar and creosote from forming inside the flue. A stove thermometer as previously suggested is great for monitoring the heat within the flue. It could also introduce CO2 into the room as the flue would not be hot enough to draw all the gasses up the chimney.
The wood always needs to be dry. I would never burn wood that is not kiln dried or been logged for at least two years or a minimum of three years for pine. (or best to avoid pine altogether due to the resin content.) I also avoid manmade timber board or painted wood as glues and paint can contribute to excessive soot build-up
Use a powdered flue and chimney protector regularly to stop creosote and tar forming. (I use Stovax Protector which is excellent)
If you are able to form an alcove around the stove line it with solid material. I have polished granite salvaged from building works. It provides fire protection but also retains heat and acts as a radiator. You might consider building a masonry alcove and chimney outside the timber structure which would avoid any concerns about overheating any timber and not take up any valuable floor space. The stove will need to be mounted on a solid hearth.
I would guess that a budget for installing a stove is likely to be around £1,000 even with an element of DIY but judging from my experience worth every penny and makes the expense spent on providing the building worthwhile and probably would make it 75% more useable.
