I agree about selling the containers and building your own booth. Cutting out the inner walls and patching and supporting the ceiling are going to be major problems..then you have length and humidity and air flow to deal with. Its easier to build from scratch than to deal with those issues.
Co-incidentally I just happen to have built my own paint shop recently.....
I built a 16 x32 because I could span the ceiling with 16' 2x6s. Then I put a ridge board up top and cut some 2x6s for rafters. I put a single vertical 2x6 upright bolted to the ceiling joist and glued and screwed to the roof truss. I simply made some small plywood rectangles and gluded and screwed them to the vertical brace and the rafter. I've done that on a previous small building and it held a 17" snow load.
In the ceiling I built several 2'x4' boxes with plywood on top of them to hold flourescent lights. You can find used f-lights cheaply. Put some of the new LED bulbs for use with F-light starters in place of the original bulbs. Pretty cheap and lots of light. On the top side of the boxes I put 2x4 plywood and then sealed each of them with spray foam cans.
I also put an opening in the ceiling for a downdraft air intake.
The walls were built with 2x6s so I would have room to make boxes for f-lights in the walls. These must be sealed to prevent outside air from entering and bringing dirt or insulation into the booth. On the front of the building I built an extended area that houses an air compressor and a breathing air pump. The 2x6 walls are covered with metal outside, then I sprayed an air barrier of foam on them before finishing the inside of yhe booth. It took 2 tanks closed foam off ebay ($600 ea). Then I put 6" fiberglass batts in the wall and drywalled overthem, again sealing around the openings with silicone caulking. At the back of the booth I installed two small drums thru the walls.
This allows me to put the drum lids on and keep cold air out, or remove the lids and draw air out of the building.
At the front of the building where I have the double front overhang housing the compressor, I built an air intake into the ceiling. Drawing air upward tends to carry less dust and bugs. There is a filter and then it enters a plywood duct I built and flows to the opening in the ceiling inside the building where it passes through another filter before being pulled downward into the booth.
I use it for storing large and easily moved wheeled items. That way if I want to paint, I can quickly remove stuff and have a nice space for painting. When I use it I plan to use some of those clips (stainless ones) like you put on open bags of potato chips to hold
sheets of plastic dropcloth in front of the lights and keep any overspray off them.
You have to be concerned about the paint fumes exploding if they get a spark from the lights, so proceed at your own risk. Maybe use water based paints.
