I built my own but I am not sure how useful I can be, my design is somewhat different than the conventional barrel in barrel. If you are interested, I posted a bit about the build plus some photos that you can see by following this link. If you have trouble viewing them let me know and I will PM you.
https://www.garagejournal.com/forum/showthread.php?t=369770I like larger water capacity boilers as there is more of a "flywheel" effect but I think you are on the right track by not over-sizing it, at least not the firebox. Smaller intensive fires are better than larger lazy fires. I doubt you will be able to re-ignite the exhaust gases. Conventional boilers just don't seem to get hot enough. I have not been able to do it, and my firebox is completely firebrick lined. (although admittedly my air system totally sucks and is now very difficult to redesign) I do recommend stainless steal as a baffle plate, I used 3/16 and do not recommend going that thick. The thing is a pain to move for springtime cleaning. Go 1/8 or less. I also have my chimney exit from the front and have to be careful not to bang my head on it when loading wood so that is a "watch out." Nothing wrong with your build and it should work fine but think of it as training wheels. Next build start to push the envelope. The guys at Hearth.com seem to have ideas on some funky designs -LOL.
I am starting to think of build 2.0. It will likely be a downdraft gassifier, with both vertical and horizontal fire-tube passes, and ideally built modular so that if there is a leak. a HX can be removed, perhaps replaced by a spare, and then repaired. But that build will not happen any time soon.
Above all - keep us posted.