I have an HTP GL-80 non ASME version like seen here:
http://www.htproducts.com/literature/glbrochure.pdf
If I had to pay full price for that, I would not have gotten one... I would have either made one from a 420# propane tank or not used one at all. I found it for $300 in Uncle Henry's down in MA. My buddy had to run down to the area so it was a quick day trip to pick it up.
I know most manufacturers suggest a buffer tank, but it really depends on how you run your house. Kedel advertises no need for a buffer tank, period. The regulation for firing rate can be sped up or slowed down so the boiler reacts quicker or slower so it won't overshoot output temperatures, etc. They also have a curve you can build that I know lots of Kedel owners have been using. For an extra $30, you can hook up an extra temperature sensor and put it in your house. You can build a curve from say 69-71*F and limit the firing rate the the house zone is always calling for heat, but the boiler will regulate firing rate based on house temperature. This allows your boiler to run longer without a buffer tank, it just varies firing rate (and thus, temperature going to your house) based on the current temp in the house. This sensor is sensitive within 0.1*F so you can easily get all 7 points built in within a 2*F curve. There is also a time average you can vary so it won't panic if someone opens a door and the sensor gets a blast of cold air, etc.
On top of that, there is a seperate loop for DHW heating, so if there is a call to make DHW, it will change it's settings to something more aggressive to make your hot water. When the water is made, it will return to the slower house heating settings.
Just food for thought. I don't have much experience with other residential boilers than my Kedel, but I did extensive research before making my purchase. I had never heard of Pellmax until now. I just watched the video and my first reaction was it was LOUD! It's as loud as my oil boiler, my pellet boiler is virtually silent. All I hear is the clinking of the pellets falling in. I saw on smokelessheat.com that the unit costs about $6300 when you add the boiler and burner together. That is roughly the same price as Kedel, Harman, and Windhagers I believe.
Just so you know, you have to use Efficiency Maine's own installers to qualify for the rebate. Even though I hit the retroactive date, I did my own install so I was ineligible.... bummer, the rebate would have paid for the install plus some extra too.