This is exactly what I'm doing, except I'm putting the boiler in my garage (not a separate building). I've already installed a heat exchanger in my ductwork for the furnace in the house, and ran the 1" insulated PEX lines underground to the garage. Just have to hook up the boiler in the next few weeks. I'll be using a hydronic shop heater to heat the garage itself (the hanging type with fan).
I was going to purchase a used standard oil boiler and convert the burner using the CK kit, however I just picked up a Triad vertical boiler with a Clean Burn burner off of craigslist. So I'll be using that now to save the headache of converting a standard burner.
In the house, I'm adding a second thermostat and will have the fan only wired to it with a "strap on" aqua stat wire in series to the fan. So, when the thermostat "calls for heat" it will just turn the fan on, unless the supply line isn't hot enough, then nothing will happen until it gets cold enough for the regular thermostat to turn on the furnace normally. I also may wire in an alarm to the aquastat to alert me if the boiler gets below temp...
I'll be getting this project finished up in the next few weeks and will be posting some pics of the setup.
Austin