Another suggestion on outlets from my perspective of being an electrician, Run all your outlets in conduit at 42" and set 4 11/16" boxes with doublegang industrial covers at all locations. This way you can change anything you want at anytime.
It does require surface mounting the conduit, and you would need to be "OK" with seeing the conduit but the flexability is far and wide better than a permanent install behind the walls with "Romex".
Also drop the sheetrock and just put 7/16" osb all the way around and you can hang anything anywhere you want.
Of course these are just my humble suggestions for "Your" barn.