For horror, my favorite will always be S.T.A.L.K.E.R. Shadow of Chernobyl by MoozE.
The eerie vibe on most of the tracks are just perfect.
And it's free > http://soc.stalker-game.com/?page=soundtrack
If the mod was built for 1.0004 then it will not work on anything later than that. Have you tried using regedit to lower the version of the game prior to installing the patch.This link showcases how to downgrade the GoG version of the game, but it seems like a similar method may work for the other versions of the game as well. You can also find working links for some of the patches at the games official website, and you can get the 1.0004 patch from here.
Start with vanilla Shadow of Chernobyl, patch it with patch 1.0005 or 1.0006. http://soc.stalker-game.com/?page=patches and do this trick http://steamcommunity.com/app/4500/discussions/0/648812305234488964/ Than the game should be ALMOST bug-free. Enjoy. Than Play CS and COP in that order. Clear sky is worst stalker game but it's still worth completing, it explains the story pretty well. Than enjoy Call of Pripiyat. After you finish the games mod it if you want. Best difficulty to start in my opinion is Veteran with crosshair turned off. Once you play it once you will only play on master - that's the best stalker experience but in first blind playthrough it's maybe too much.
The game is already installed? Then you could try some of the patches. I'm not sure if they need to all be installed sequentially or if they are cumulative. You can find them here: http://soc.stalker-game.com/?page=patches
At the moment I use the GREAT stalker soundtrack while playing Fallout 4 - it just gives a much more dystopian horrific vibe.
(The soundtrack is available for free on the stalker website) http://soc.stalker-game.com/?page=soundtrack
I don't remember any zombies in the X-18 lab... Do you mean X-16 perhaps?
BTW. Playing with Autumn Aurora 2 too :)
EDIT: Or maybe you're talking about snorks?