So my thoughts would be:
1. Powered cases
2. Unpowered cases
3. Power supplies

I don't think it would be worth it to have cases available for building a virtual rack. It falls into the "would be nice" column.
But having the information available to peruse would be awesome. I would include a main picture of the case. Then I'd allow users to upload their own custom pictures of the case (with or without modules) to add to the description.

The description should also include the number of rows, the HP of each row, and the size of the row (3U or 1U) and the format of the row (useful for 1U set-ups since 1U isn't standardized).