Trying SH3 for the first time. Have some questions.

[sickpeople]

Hey everyone!
I'm a long time SH4 player and I really want to try SH3, since I've never played it and everyone says it's the best U-Boat sim ever.
Before I do, I want to know if should I play vanilla to get the hang of the game mechanics or install a megamod right away. I'm a bit of a purist and I'd like to try out the game first as it was meant to be by the developers, without overhauling it too much, if it's possible.
Also, which mod should I try out first once I get the hang of vanilla? I'm a bit of a casual player and like to have a fair challenge that's historically accurate and authentic.