If you delete a file, empty the recycle bin and then do a freespace erase the files are gone for ever provided the erase completed successfully.
Erasing an individual file will just erase that file and not previous versions of the file deleted by other means, hence the need for an unused space clean.
If you have an undelete application installed which saves your deleted files then yes they can be recovered as these are not deleted but moved to a recovery database.
I have heard of it, I know nothing about it, Data Recovery is the one I see recommended the most. I had to use Data Recovery once many years ago on a family members machine and recovered quite a bit, but it was a purchased version.
Try the Data Recovery and see if it finds anything (what have you got to lose? as of now you have already lost data) I would try the trial version and see if it recovers anything before thinking about buying the full package.If it does not recover anything then I would not buy it. Maybe then you might try the Data Recovery trial and see if it can recover deleted files from trash.