Normally the advice is to play stock for awhile, since this will make you appreciate just how much of a difference the mods make. Since you are interested in the u-boats, I would say that unless you want to play u-boats in the Pacific in 1943 get Operation Monsun right away. It will let you run a full u-boat career from 1939, in any type u-boat you want.
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“Never do anything you can't take back.”
—Rocky Russo
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