If I had your experience I would run far away from any boiler with built in storage. You should be able to install a separate boiler and a couple insulated propane tanks for similar price. You would then have pressurized storage that will last for far more than 25 years. A Garn or Switzer puts all your eggs in one basket so to speak.I just priced the Switzer 2150 gallon one is around sixteen grand and the 2600 gallon is seventeen grand. By the time I ship it and redo my building to put it in and get it hookup I sure I would be at twenty five grand. That's a big investment for a older guy.
For initial cost savings you could no doubt find a used gasser as suggested earlier.