I have been researching this myself. Check out http://www.hearth.com/econtent/index.php/forums/ for alot of great info. One thing they really push is not going cheap on the insulation from the boiler to the house. The recommended option is to dig the trench and then spray foam the pipe into the trench. The guys on the forum say it is the best investment you can make in a system as you dont lose heat to the ground.

Another thing I have read is that gasifiers use the least amount of wood and produce the least amount of smoke.

I dont have any experience with this stuff so I am eager to see where this thread goes.