There are dimensions for egress windows which are not met by many older homes. 

Perhaps we could squeak out a higher profit by doing a build-out but that will take longer and add a whole new dimension to the risk profile. 

Will I be able to see it first in order to determine the dimension and cost of the rehab that will most likely be involved?

My uncle told me that he knows a shipping container builder that can make containers of any desired dimensions.

I saw some wire cage structures, but they are much more expensive ($400 each for a 4'w x 5'd x 7.5'h), I had a fence company to quote me $2000 for four units of the same dimensions listed above.

Here is how I like to do it:Find ~10 properties in the same neighborhood with similar dimensions, square footage, age, amenities, etc.

But I also don't want some toy software for rough designing w/o accurate dimensions, etc. 

They were very rigid about the dimensions of units they were trying to fit into the lot.

You've gotten some advice regarding the 12' length, but this dimension is not your problem.  

Providing visual aids, photos of other properties could help in communiting your design ideas.Ideally you also want to discuss and define every little detail of the project for every area of the house/apartment, positioning of every light and plumbing fixture, placement and dimensions for areas that need backsplace tile, pocket door installs, reframing and expanding certain areas to increase spaces for showers, living rooms, closets, new shelving to be built, opening up ceilings, re-routing plumbing, repositioning toilets, etc.Then you hope that your ongoing communications will work well.