I'd start with whats been said above but as you get better you'll start to get more into the "simulation" aspect of the game and then maybe some more of the less cosmetic shortfallings of the game start to nag at you. Then I'd suggest either Real U-boat Mod or Improved U-boat Mod. THey are mods which attempt to fix all the flaws of the game in one package. RUB is more of a hardcore realism mod while IUB incorporates much of RUB while still being leniant to us less obcessed Kaleuns.
Its also alot to do with testing stuff out and seeing what you like. I change alot of things. The IUB I use isnt really IUB considering how much stuff I've either added or removed.
Also SH3 Commander is definately a must have regardless of what you're using. Also the Hollywood Advanced mod add a few interesting Fatigue mods which you can use through SH3 Commander which I feel better represent real fatigue than any other available fatigue mod. But again my opinion. Its all about finding your preference.
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