The most important step in deciding what system to use is a heat balance calculation of the heat loss (winter time) and the heat gain (summer time) if air conditioning is being considered. This involves the square footage, cubic volume of the structure, insulation values in external walls and ceiling, windows, doors, building orientation, and the climate zone its located in. In addition, frequency of the opening of the main door way to factor in infiltration of cold/hot air the system will see AND what interior temperature you want to hold.
The second important factor is operation....electric/heat pump/natural gas or propane fuel or oil or wood. Condiderstion must be given to the possibility of explosive vapor being present (gasoline, kerosene, paint fumes). If so, local codes MAY require a sprinkler system to be installed, based on square footage and building construction. Do your own due diligence, rather than acceptance of what your "buddy" installed, who in all likelihood did not factor in any of the above mentioned issues, but used scrounged components. A radiant heat tube system or a heat pump or a sealed combustion furnace offer the best alternatives to consider. Just my opinion of course.
BOB RENTON