The best thing I ever did was install a a/c and heating unit in my barn workshop (I think it is a Mitsubishi):
I think is it 16k btu. On the hottest, humid summer days it really takes the edge off and allows me to work on stuff sweat free (I also augment with floor fans). It is not a split unit but I really don't know the merits/weaknesses of this unit over a split unit.
Winter here in Maryland can be a bit nippy but the unit allows me to work year round, which is great since the winter is my equipment maintenance time.
I also have a small window unit in the separate work area I use for small, detailed jobs so I don't have to heat or cool the whole shop.
The shop is slightly bigger than a large two car garage. I did insulate.
Two wrenches up.