If you are going to put an unfired boiler in an unheated building for and not use it at least every other day, then I would say antifreeze would be the only way to go. 30% solution of propylene glycol wholesales for about $4-5 a gallon in 55 gallon quanities. With 5 gallon pails it can jump to $15 a gallon due to handling and container costs. You will have to find someone to purchase the glycol through and expect to pay a markup, tax and possibly a delivery charge.
The chemical companies predilute the glycol for you to ensure no on site water impurities affect the solution. If you need more than 55 and less than 110 gallons, buy in a greater strength and dilute on site with distilled water or water that has gone through an reverse osmosis machine.
30% will freeze proof the system to 3 deg. F. and burst proof the system to -10 deg. F. Even if it is that temp outside, it will take days for the building and system to reach that temp.
Dow Corning and Interstate Chemical Co., Inc. are the two major players I know of in HVAC gylcol solutions.
Other options are:
1. Get a stoker furnace or stove in lieu of a boiler. No water to freeze.
2. Get a thermostat that allows you to set the building temperature at 40 deg. f. when you are not out there. It takes very little coal to keep the building at 40 on a yearly basis if it is somewhat sealed and insulated. I put over 100 gallons of glycol in my system thinking the same thing as you. I got tired of re-lighting the fire and wanted to leave the plumbing operational during the winter. The second year I left the thermostat at 45 degrees during the week. Most of the time, I was burning about 15 lbs a day just to keep the fire lite. Only in extremely cold weather did I burn coal to keep the building from freezing. I found I was burning just under two tons a year to keep my 2400 square foot workshop warm with only turning the thermostat up on weekends.
Glycol will keep your piping system clean as it is slighly agressive. Any leak that would normally self seal with plain water will continue leaking with glycol in the system. You must have perfect joints or leaks will be an issue. Also, the gylcol should be tested every year to make sure it has not lost the inhibitors and become corrosive.