Don't purchase a outdoor wood boiler unless you're at least 1000 feet from your nearest neighbor. Multiple law suits being filed across the U.S. against OWB owners for nuisance and health issues. Not only will you be ordered to shut down and lose your investment but you'll have to hire an attorney.
Another problem is many local communities enact ordinances shutting OWBs down. Again you'll lose your investment. Lastly, some of these devices require anywhere from 10 - 20 full cords of wood per heating season. Ever cut 10 - 20 full cords of wood? Add to that the requirement to only use dry wood, you'll need to cut up to 40 full cords initially to make sure you drying one year in advance. Do a search on trouble with central boiler OWBs and you'll see the multitude of problems with these things and CB many times finds excuses not to honor the warranty. Don't believe any of this just do a google search on any of the mentioned topics.
Many states are also considering banning OWBs. All in all it's just not worth it. Purchase an indoor EPA certified wood burner instead.