Those containers are butt ugly, but they certainly are a quick fix for getting enclosed storage. I find the door locking mechanism is a hassle to deal with. Since what I store does not need such level of protection, I plan to cut off the door locking system and just put on a couple handles with a simpler padlock. Also, the door bottom is below the floor (~1.5"?) and the bottom of the latching mechanism is below the door bottom. Therefore, you need to keep the ground under the door swing at least four inches lower than the floor if you keep the locking mechanism in place. This is only a concern if you are storing items with smaller wheels and a bit of weight. I have to use a board as a ramp for many items.
Ventilation is a challenge. Use caution if putting an attic fan into the roof, since it will likely leak. A side mounted unit will be easier to keep rain out of the container. If you don't have electric, a smaller solar-powered fan might be another way. Just be sure to cut in a vent on the opposite side for cross ventilation.
I see many have placed these containers right on the ground. That seems like a bad idea. Clean gravel will work (thick enough to stay gravel and not let the dirt work its way back up and into it over the years). Or railroad ties. However, any foundation not supported from below frost line will heave and move a little each year and could be well out of level over time.
Modification for openings will require a lot of drudgery with a metal cutting saw. A torch will be faster or plasma cutter.
You can drag the container around if needed, but they are very heavy. If you don't have a good size tractor, you will not be able to lift them onto a foundation. I think around 5,000# empty? I can check if you want. My 40 Hp tractor with FEL could not lift one end. I had to use my much larger skid steer.
I put four of these containers in a row. I will be putting a roof over them to make the set look like a regular building (did I say how ugly they are?). And then painting to match. One is for household storage, second for all my lumber, third for 4x8 sheets of plywood, etc. and all my various metal. Fourth for motorcycles and other play toys.