All are very good ideas, how ever i think that another option would to format the Drive, then reinstall the 0s, as you don't know if the person has installed a back door of some type.?
So to make sure that everything is gone, as some programs might miss something or might not detect something, then probably a format of the drive although it take's the longest still might be the best option..

Just a thought anyhow.

cheers
..::front2back::..