|Keep It Simple, Stupid
Also, you'd be surprised what has been documented. I often find myself doing a Windoze search for documents containing any of the names the project went under over the period in question (system names can change over time). Also check for whether at any time the software has been kept in a version control system such as cvs on *nix or SourceSafe on Windows.
It might be possible to fill in missing links in its evolution rather than attempt to reverse engineer functional and technical designs from scratch which, over ten years, might be impossible.
I might be telling you what you already know, but just in case: if it turns out you have overstated your goal, e.g. someone pulls out a pile of documentation and says, "what do you mean? Here it all is!", the business case for the time you spend on a full reverse engineering effort is liable to implode with collateral damage.
^M Free your mind!