Documents - retrieve older version

This depends incredibly strongly on what type of document it is, and what sort of "compare" you hope to accomplish.  Years ago there was a plug-in that does a rudimentary analog of the "compare versions" availalbe in MS Word, though it had certain drawbacks - I believe that plug-in is still generally available though I haven't checked.  If you're talking about other document types, though, you might be completely out-of-luck unless there are newer plugins available that I don't know about (which is possible, you should check), or alternately, you plan to write your own.