Shipping container homes have a real draw for me. I think it is the fact that you are reusing a structure that might just sit idle or stay rough as a storage container.
The structure of a shipping container is strong. However, when cutouts are made the container can loose it strength so reinforcements usually need to be made. Since it is just a metal box you need to insulate it. This is done with an interior wall with insulation behind it.
I like this design by Pin-Up Houses. Using two small 20 foot containers joined together you have a nice space big enough for a living area, a bedroom, bathroom, kitchen and even a designated storage area.
If you do the work yourself the construction costs will be quite low, around $24,000. If you don't have the skills you could also contract them out.
Your biggest expense will be the land and a simple foundation. You will also need to tap into the local utilities if you are using this a an ADU (Addendum Dwelling Unit).
If you are off-grid setting up solar should be relatively easy with the flat roof to tap the sunshine.