I know you're trying to set this up on the cheap, but because the building is separate, I would still run a glycol mix through the system.
Even though you have a separate heat source, if the electricity goes out in below freezing temps....
The unknown is what effect the glycol will have on PEX made for domestic water supply.
Also, I wasn't promoting solar in my previous post
, the link was for you to enter the values of the building and come up with a BTU loss number.
There are other calculators if you google for "heat loss calculator" or "BTU loss calculator".
Also, the crimping tools (rent or purchase) and hardware for a pressure test are all available at HD.
Mitch