Welcome to SH4, ruskoz!
If you've not played SH4 at all, I think I would personally play at least a few patrols with the stock version. Once you're comfortable, and if you like to aim for realism, I'd highly recommend Ducimus' Trigger Maru mod, which changes a ton of elements within the game. Most notably, the AI and their sensors, making the game significantly more difficult. Along with the TM mod, I'd recommend the Run Silent Run Deep mod, which changes the missions you're assigned and makes them more realistic and in-depth. The Natural Sinking Mechanics mod is also excellent, which seeks to have ships sink (or not sink!

) more realistically. To top it all off, grab some good gramophone tunes or get a radio station mod to set the mood. Their are, of course, a ton of other mods that are just as good, most of which I've not used. Hope this helps. Enjoy this fantastic game!
Howler