Worst thing when working in a container is the ceiling height which I find oppressively low. If you are uncomfortable in it you won't use it.
I wonder if lightweight add on roof and side cladding suitably spaced from the sides with the space ventilated might make a useful difference to the internal condensation issue. Tent fabric ought to do for a short term job. As I understand it the proper office / workshop conversions are a bit more sophisticated than simple linings. I have seen a similar conversion, lorry body in this case, with simple plywood lining that had rotted out after only a couple or four years. Basically a pond between lining and wall.
The ex army workshop breed will be very expensive and well used. Generally they don't get sold off until the interior needs stripping out and re-doing. Despite decent windows I still wouldn't care to do standing up jobs in them. Sitting at a bench for electronics work was OK (ish) except for the pressure of getting failed trials equipment up and running without wasting expensive range time!
Time for a lot of Googling for ideas on how / what to do and pitfalls / "I wish I'd" posts.
If you aren't careful this could easily turn into an expensive cheap option where starting with something that looks as if it's going to be less costly ends up being both more expensive and not as good as doing it right. This was certainly the case with the two or three similar conversions, albeit lorry bodies not containers, that I have personal knowledge of. In both cases the costs and effort of all the detail bits that didn't get thought through exploded.
I wonder how expensive and how practical it would be to make a timber shop built the way mine is easy to take apart and re-assemble. Mine is 2 x 4 framing with OSB panelling inside and out. Insulation between the panels and shiplap cladding outside. Insulation above the ceiling, floored loft – store and insulated steel sheet roof. In principle it ought to be possible to come up with a way to join the panels together without needing to get inside them. Perhaps something as simple as (stainless) steel strips and angle screwed on at the joints would work. Major Issue would seem to be pulling things up tight and square before fitting the strips.
Clive