Just sharing this in case it's of use to someone. I published a guide to how I built my first wee hut on wheels. It is on the "Instructibles" web site here:
Note that the prices listed were 2012, so some things have gone up in price since then. Time wise, about 400 person-hours to build. I am quite careful and methodical though. At the moment, it is costing about £7,500 for the materials to build to my current spec. It is a lot of money, but I like to use high-quality, custom-sized windows and doors which are time consuming to make and are insulated and double glazed with safety glass. A lot of cash could be saved using re-claimed/salvaged windows and doors. I also use waterproof, construction grade plywood and veneered ply which is expensive. More savings could be made using cheaper sheet materials. Good quality wood burning stoves and twin-wall flues are very expensive too. I have been wondering about whether corrugated steel is a cheaper wall cladding material than timber? I'm not sure, but I prefer wood anyway. Caravan chassis are relatively cheap, but have lower weight capacity than, say an old Ifor Williams trailer. Some folk use ex-agricultural trailers which can be very good value for money (at auction perhaps?) and can carry massive loads.
Hut with woodburner and 240V electrics ( caravan style hook-up)
Guttering is good for keeping cladding drier.
Engineered, insulated, double glazed, custom size (small) door set from local joiners.
Interior with 12V solar lighting.
Burley Springdale stove, twin wall flue. Solar lighting.
Solar control unit (which can also charge a phone or tablet via USB). From "Sunstore", who were very helpful with advice & specs.
12V solar panel, which charges a "Leisure" battery (hidden by panel), housed in a box between the A-frame rails. You need a controller to keep battery in good condition and manage the charging, but they only cost about £20 upwards. Note; Good quality, weatherproof 12V cable is expensive, because it will carry a high current (low voltage = higher current for the same power, compared to 240V) Don't use skinny wires! Always install fuses. Take professional advice! A 12V battery is quite capable of melting and burning cable during a short-circuit. You can melt & weld steel with a 12V battery. They are powerful energy stores.