And yet all it took was to look at Paradox forums. :-)
Sorry for the late response, but many thanks for the uploads!
Regarding the Paradox forums, I did actually spend some time trying to find it there but the place is a mess. It has a whole subforum dedicated to the CORE mod, but when you open it, it's completely empty, saying "no threads to display". The board entry leading to that subforum does have a link to the last post inside and strangely enough, that does open a thread about the CORE mod, but all the links in there pointed to the old stonyroad.de fansite which is now dead.
Having browsed through the place again just now, I noticed there indeed is a mod mirror collection thread inside the Mods subforum, but since there are no other threads on that board either, it's easy to miss, thinking it's just another empty section like the rest.
As for what this mod does, it's like a massive expansion that adds more of everything. New units, new tech, improved AI, reworked unit stats, new graphics (unit sprites), new nations, thousands of historical events, tons of historical generals, etc. It's a whole new experience that makes the game a lot more realistic (and harder).