Hello,
My GF loves telltale's episodic "adventure" games. Well some of them.
As we're all aware those games, especially The Walking Dead and Wolf among Us are more like
"chose your own adventure"-type fare, but in this case that is exactly what I'm looking for.
How Dreamfall chapters compares in that regard?
Is it a "classic" adventure game, or is it strictly choice/story based?
How engaging would it be for newcomers to the series?

Dreamfall is something between Telltale's adventures and classic Point and Clicks.
While you have similar presentation of conversations, with choices that matter, Dreamfall has no quick-time-events or action. Conversations aside it feels more like a Point and Click adventure, where you can roam and explore the world and have to solve some puzzles (which are generally easier than in classic P'n'Cs).

Immersion and also story telling is great and I can recommend the whole series. Actually I'd start from the beginning to get the best of it, but it isn't necessary. Though the predecessors of Chapters are much more Point and Click orientated.

However, while Dreamfall Chapters has some parts in common, it isn't exactly like Telltale games, so I'm not sure your girlfriend will like it. You aren't driven through the game, but have to put a bit more effort into exploring and puzzle solving, which makes it a bit less casual and the thrill of quick time events is missing.

Red Thread Games has taken Telltale's forms of interactivity - conversations, hubs, plot choices - and improved every one of them. There are two really big hubs in the game as of now where Telltale just renders diorama like small environments. If you like, you can really squeeze a lot of backstory out of DFC's characters. And although that was the direction RTG originally wanted to move into: The choices aren't so ennervingly artificial dillemmas. There are often rather easy choices like in Mass Effect. And no quick time events is always a huge plus. :)

In my opinion, Dreamfall Chapters wipes the floor with Telltale's (present) games. But you will need considerably more time and patience for Chapters than for Telltale's games. This is not a bite sized, fast paced, actiony-animated story. It's a true epic with a ridiculously large cast of characters and a complex backstory that's at times not even too transparent for its most avid fans. It's a game that asks of you to memorize two city layouts without any kind of mini map. You_will_get_lost in those for a while.

Where Telltale still excels, of course, is facial animation and animation in general. Chapters has stellar voice acting, but hardly any facial animation; conversation is usually shown without any animated movements as well. That doesn't mean RTG can't pull off impressive animation – when you e.g. find yourself walking in a toddler's shoes, RTG's animators have their day in the spotlight.

Telltale's seasons starting from 2012 will take you six to ten hours to complete. Chapters will probably take you 18 to 25 hours once it's completed.

There are elements of the classic adventure game thankfully, but not nearly enough for my taste. It's often just about finding the right hotspot, merely looking at an inventory item, combining two very obvious items, or sending an NPC in the correct direction. In very few cases, most of which I love, RTG goes beyond that.

The story is definitely confusing for newcomers, but it's also confusing for those who have enjoyed the series for 17 years! RTG makes a very decent attempt to accommodate newbies - starts very slow, introduces a lot of new characters, gives you a three minute summary video accessible from the main menu. It worked well enough for a friend of mine (who played both prior parts, yet nine/ten years ago).

But if the backstory of a recurring character just intrigues you, and you just don't want to play the older games, you can always consult the TLJ wiki. We're making this thing exactly for people like you, I guess. Well, and for the developers who don't see quite so straight on some things they concocted two decades ago. :)