My small detached 14 X 26 shop building is insulated with 3 1/2" fiberglass in the walls and 6" in the ceilings. Heating and cooling is provided by an 18,000 btu window style heat pump mounted high through the North end wall of the shop. It has proven to be a safe and ideal way to heat and cool my woodshop, but with only one problem. A good pleated type air filter is absolutely necessary to keep sawdust from plugging the coils of the unit DAMHIKT. I've found that a 12 X 24 X 1" standard size filter will fit between the front cover and the coils of my unit, and this works very well, but needs to be cleaned and/or replaced frequently. Mine kind of also doubles as or replaces the need for a ceiling mounted shop air filter. If you go with a mini split system you will have the same need to filter the air going through it very well to keep the coils from plugging. The coils can be difficult to clean if they get plugged, so make sure that the unit that you buy has the capability to use a good filter that can be changed/replaced easily and often.
Charley