You may want to view the thread, Keystoker or Harman which I started a month or so ago. I researched all the available choices, and finally made my decision for the Harman.
I picked up the boiler this last weekend so I have no experience with it burning, but I can tell you that I am very impressed with the quality of construction. The boiler is 1/4" steel plate contstruction. Most of the welds appear to be machine made, and fit and finish of the unit in general seems top notch. I do have experience with the Harman stoker which is the same as the Magnum and is very well built. Holding a fire for 3 or 4 days with 250# hopper shouldn't be too tough unless the weather is very cold.
In your first post you stated that you wanted to keep your budget within $7000 including installation. You could certainly do this with the Harman or a Keystoker, not sure thats doable with an AHS, Axeman, or EFM boiler.
Lastly, on chimneys, I know most people would reccomend a masonary chimney, but I would also consider one of the better insulated metal chimney systems. Interior masonary chimneys are fine, but exterior ones tend to run cold. Cold chimneys do not draft well,and coal boilers, especially the newer ones do not have high flue temps, especially when they are coasting in warmer weather. The Harman boiler installation I visited had an exterior block chimney and the owner had to use a draft inducer to help it along.
I would look at chimneys with a higher grade nonmagnetic inner liner which are more resistant to corrosion.
Good Luck with your decision.