No mod is "necessary" in SH4. The v1.3 Steam game is somewhat lacking, and you cannot "update" it with the Ubisoft v1.4 patch, but if you could, that would be the minimum version number to run. The v1.5 of the game has the U-Boat Missions Add-On, and it the latest/greatest version, and is "the best", though it is already 16 years old... Check the FotRSU thread for comments on your issues there. Just remember that you cannot apply ~ANY~ mod with the game in the default install path (Steam: "C:\Program Files (x86) \Steam \SteamGames \Common \Silent Hunter..."), since Windows will "protect" the computer from the destructive force of the User. See
Moving a Steam Installation and Games or
Changing the default install folder