here with some newbie questions.

[Lapeer20m]

I have seen large quantities of used antifreeze on craigslist for free or cheap. You could also try to find it used at scrap yards.

Don't use more than 50% as antifreeze does not transfer heat nearly as efficiently as water.

 

[Lapeer20m]

I understand cheap!

One place where you "need" to spend money is insulating the pex lines between boiler and shop.

The best solution is digging a trench and spray foam the pex in place.

The next best solution is to run above ground inside an 18" culvert packed with insulation.

 

[Lapeer20m]

I don't believe that a person has to know what they are doing to build this.

They need to know how to weld, and they need to make sure it's an open system.

The OP has both of these bases covered.

Worse case scenario: the draft control malfunctions causing the unit to run wide open when nobody is around, boiling all the water out. Then it becomes a wood stove and radiates heat to the atmosphere until all the wood is gone.