I currently own a 20foot can. First thing I did was install a workbench and shelving down one side and across the end. The other side has hooks for hanging line, fenders, sail bags, anchors etc.
Second thing I did was open up a vent in the left side door and put a grill
over it. These cans are pretty water
and air tight so any moisture inside in hot weather
will turn it into a terrarium. Lots of mold
if that happens. I spent one winter wiping down lots of stuff with lysol after I sealed up the can in june and didn't open it again til august. Sadly there were a few leaks
got it. It was like walking into a jungle. Gross.
Third thing I did was spray paint
the inside and outside gloss white. White insides allows light to bounce around, so with the door open at mid day,, its very well light inside, even at the back end. The gloss white exterior helps keep it cool.
I suggest that if you are going to use them as shops, build an awning frame over them from scrap material and put awnings over them. Keeping the direct sun off them will do a great deal towards keeping them cool inside. My can is in a yard so I have one can stored on top of mine. Instant sun roof.
One thing you might want to consider is insulation
on the outside. Put 2x2 strapping on the outside with foam or glass batts in between, then sheath with what ever. Installing doors and windows is not difficult either, an abrasive saw will cut them pretty easily, then frame up the window or door with flat strip or 3x3 angle iron.
I'd say go for it, you certainly can't go wrong with them. I've even moved mine a couple of times without emptying it out. I just put plywood
across the shelves and parked everthing else on the floor. No problems.