Quite a few years ago I worked for a company that bought old shipping containers and refurbished them to use as rental storage units. We also did quite a few as portable worksite storage and office units. As you found, these units are vented on each corner, but it is minimal ventilation and was in place to prevent moisture problems inside the containers.
We used to cut in windows and doors all the time. We would cut out the opening, then make a frame from rectangular tubing welded to the corrugated sides. Then the doors and/or windows were attached to those fabricated frames. Barred windows were a standard item, and doors were always steel and were set to open OUT. A secondary sliding door made of heavy bar could be closed over the outside of the entry door, and the padlock was inside a pocket where the hasp could not be cut, much like "Job box" tool storage boxes. The beauty of this arrangement is that (1) a door that opens out cannot be kicked in, (2) space inside the unit did not need to be wasted to allow room for the door to swing in, and (3) the sliding security door was built heavily enough that it was NOT coming off.
Most job office units were forty footers, split into a 20 foot office and a 20 foot storage area, separated by a steel wall that was welded in. The rear end with the doors was obviously the storage end. The office door and window(s) were added on the sides only, because the hookups for power and phone were on what had been the front of the container, along with a heat and A/C wall unit similar to those used in many motels. Units that had heat but no A/C used electric baseboard heat instead of a wall unit. Walls and ceiling were framed out with 2x2 studs in most cases, though some had 4" thick walls, styrofoam insulation, and white plastic crinkle finish paneling. Floors were also framed out and insulated the same way, but with 3/4" plywood added as a subfloor on top, covered with plain old linoleum flooring. Modern practice is to use self expanding urethane foam for insulation so that moisture cannot get to the steel walls, as they would be completely sealed off by the urethane. Breaker panels, outlets, switches, and lighting are all wired and installed prior to the insulation being installed.
Extra ventilation was not added to those containers, as job site offices were generally not "heavily populated" and the door opened and closed frequently throughout the day, providing some fresh air each time it opened. In your case you might want to add some form of forced ventilation that can be turned on as needed, as there are many opportunities to create noxious fumes in a shop environment.
I realize that you probably have your plans pretty well set already, but thought that I would add my two cents worth based on what we used to do for commercial use on rental units, knowing that you might find some tidbits useful. We did make one big mistake with our first office container though. When welding in the new door frame, we forgot to allow for the thickness of the new raised floor that was going to be installed, and we had to cut it all out and cut the opening 2" taller and put in a 2" filler bar at the bottom of the opening. It was relatively easy to fix, but we were kicking ourselves for overlooking something so obvious. Thankfully we discovered our error prior to painting the container, which was normally done after all of the welding was completed, but before any interior work was begun.
I wish you luck with your project, you should find that the container will work well for your purposes.