The problem is that the surface under the backer board is either not level/flat or it is not thick enough (too much deflection).Additional layers of backerboard do not decrease deflection.

Keep in mind waterproofing  and drainage for any below grade living or storage space you are creating.  

Be patient, have your money ready to make an offer when the hidden gem comes on the surface and you have your reserves and game plan ready then you strike.

The perfect property is 60 acres or more, and a bit of a mess--perhaps has drainage issues or timber needs to be cleared, super-motivated seller who wants nothing to do with farm acreage, among other warts--in order to get a decent price.

At only 400 sq ft, there isn't exactly a lot of square footage for mold to hide in on surfaces.

Slightly more valuable properties I prefer to rent for less and save the expensive updates for just before selling because that price range buyer is more sensitive to what is in style...beige granite now dated in favor of grayer quartz whereas low end is thrilled with any solid surface over plastic laminate.

So...let's say we get list price almost immediately (otherwise known as best-case scenario), everything on the surface looks good until you realize that you have to pay taxes on the profits (bye bye, 25K), closing costs on the property (let's estimate 12,500), RE commission (6K). 

Snow tires are often not better on ice because you want more rubber contact with the surface. 

After that I saw that it wasn't as great an idea as it seemed on the surface. 

I also think it is exciting, the biggest reason being that the concept of container living is common in many different countries, and has just begun breaking the surface in the United States within the past years.