I bought a used Pacific Western, company has been out of business for years. I replaced the fan, aquastat, solenoid, and 2 new pumps. I bought it not knowing if it leaked or not, figured I could fix it if it did. I was lucky or at least so far anyway and no leaks. I've been reading about owb burners for years and had planned to build my own but this deal came along so I bought it. Mine is working good and I'm real happy with the money I have in it so far. Spend your money on good pipe, I used Logstor for one of my runs and a homemade one for a short 35' run. I heat 2 houses with mine, I installed the whole system myself including building the tinwork for the exchangers. Installed a plate and a tube heater for domestic hot water. I never did any figuring out on heat load, I just bought what I thought would work and it does. Next year I will have my shop heated for the complete winter and hopefully actually do something out there.
You might want to put your location on in case someone is local to you and you can see their system.