I live in southern Michigan and just had another pole barn built(24X34X13).I went with the spray foam insulation about 3 inches thick(ceiling & side walls)I went with a unvented 28,000 btu LP gas wall heater,it about drives you out of the barn on the lowest setting.coobie
Ventless heaters also fill up the building with moisture and rust all your equipment stored in there. I'd suggest upgrading to a properly vented model if you use it more than occasionally.