I haven't played this game yet... however, I have my eye on these mods:

Kentie's Launcher, Deus Ex v.2.0/Human Renovation and GMDX. And later Project 2027 and ZODIAC.

I really like the easy difficulty in GMDX I heard about as I know the game can be hard.

For the campaign: GMDX. It's far above anything any other campaign mod offers, and includes the standard kenties.exe, modern renderers etc.
Total conversions: Nihilum, TNM, Zodiac, 2027.

What is best for a first playthrough-noob like me:

GMDX or vanilla?

I'd say go through the youtube vids talking about what GMDX does because they're very much in-depth. If what you're looking at sounds invaluable to you take it.

Honestly Vanilla doesn't need anything, but GMDX is one of those mods that works in a way that a first-time player can use it. You can always play the original while waiting for the new version (it's gonna be big anyway.)

GMDX is incredible.