To prevent the dreaded rust fairy visiting, DO insulate everything that you can, and build in a small vent at floor level, and another high up. These will allow damp air to escape. Obviously, the outside of the vents needs to be both bug and weatherproof.
50mm of glassfibre will make things snug and minimise the risk of condensation. Many folk advise using a vapour barrier between the outer wall and the insulation.
Over all, this will make the shop warm (and easier to heat) in winter, and cooler in summer.
My small shop has no windows, so my high level vent has a small fan to help circulation; although for much of the time, the door is open, (only closed when rainy or frosty).
Howard