I just went thru this. I was going to take 3x 20ft shipping containers and space them out the width of 5 containers and cut the sides out and then weld them together to amke a single 20ftx 42ft shop. I ended up going with a pre-made building shipped from china that came with 3" insulated wall and roof panels.
The reason i chose the china route was the ability to have insulated wall/roof panels, the building looked much better in the end, and the price in the end was comparable or even cheaper than buying 3 20fters and cut them all up to just have to purchase framing materials and insulation.