I have the 6048, it’s slightly larger than the model above. I estimate that I burn around 10 cord a year and heat a 3,700 sq ft house. In addition, I get domestic hot water out of it too. That is a huge savings with 5 people in the house that include 2 teenagers.
i would look for any kind of weld cracks or leakage spots (rust) around the any welded areas. I have had great luck with mine (knock on wood). They can be had on Craigslist; I’ve seem them multiple times at a very reasonable price.
If you do decide to purchase one, get the best insulated pex. I already had to redo mine because the dealer talked me into the cheaper pex because they didn’t have the good stuff in stock. Good luck!
https://centralboiler.com/products/classic/