Welcome aboard!
First of all - definately get GWX... you'll never believe you enjoyed Stock SH3 once you get your feet wet in a GWX career. Just the new ships (there's gotta be about 50+ new ships) alone is worth it. GWX does not adjust the career so to speak, it just adds alot to the game like harbor traffic, bigger convoys, better escorts, more realistic weather, etc, etc

To go with your GWX, you really should get SH3Commander. It doesn't directly effect the game, but allows you great control over your career. Think of it kinda like SH3 the RPG...

but it also gives you options to tweak your career like adjusting the fatigue of your crew, water clarity, and others. JSGME is a program that will auto-install when you install GWX. It is simply a MOD Enabler/disabler - but it will make you're life immensly easier as you decide to get sub-skins, other MODS and turn on/off GWX's various optional MODs (16KM Atmosphere, Enhanced Damage, Type VIIc/41 playable, etc).
It sounds like a lot at first but really it starts to make sense as you get it all installed. That and there's a whole bunch of guys here that'll help you get sorted out!

Just a word of caution... make absolutly sure you follow the installation instructions for GWX - or you'll have a TON of headaches!
Good Luck & Happy Hunting!
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GWX3.0 - SH3Commander - Thomsen's Sounds Pack - Manos Env Pro - Hitman's Optics - RB's UBoat Flags - Waterstream & Exhaust CombiMOD - Torpedo Damage Final - DiD
AMD Phenom II x6, 16GB DDR3, 2x NVidia GTX460 1GB, WIN7 64bit