Structural reinforcement is unnecessary - they can be stacked half a dozen high safely. A bit of back of the envelope tells me the cost of paint to coat the exterior of a standard 2 story suburban house is about $1500, so I estimate painting a 40 foot container should be about $400 max. Insulation should be about $1K. I imagine for drainage a buried septic tank is the best solution here.

I think the cost to retrofit an otherwise sound but unwanted container is less than the cost to purchase a used one. That is orders of magnitude lower than the cost to erect "standard" solutions to the housing problem.

Another unmentioned problem is that the plywood floors are saturated with carcinogenic insecticides- and not a small amount. Source: I sealed floors for a container reuse project.