just to understand what I am up against, when a file is replaced with a newer version, is the replaced file recoverable? Or is it only when a file is deleted that it can theoretically be recovered?

When I read that the system eventually reuses the deleted space, does it first use all the free space on the drive before overwriting deleted space?  As in, an 80GB drive with 20GB of space used would first write to the 60GB of free space before writing over deleted space?