What I would like to know is if it's possible to create such a dual class and if it works, which stats, proficiency, alignment etc do I need to put points in.

I like the thought of a melee fighter with the ability to shapeshift and use healing spells when necessary.

Not without shadowkeeper. An Avenger is a kit, and you can only select kits as your first class when duel classing.

So if I get this straight I can go from Avenger---->Fighter but not Fighter---->Avenger.

If i dualclass I can only choose the 'standard' kit so to speak, instead of the specialized avenger I can simply choose druid because it's a standard class. Did I get it right?

Yes you can go avenger -> fighter

or Fighter (or Berserker, Kensai, or Wizard Slayer kits) -> Druid (no kit)

Alright I think I understand it now, thanks.

If you're like me and think that the restriction is silly, you can use a savegame editor like Shadowkeeper to change the druid kit to Avenger. I used it to dual from a fighter to a shapeshifter, myself.

I do think that it's a bit annoying to say the least.

But I do not wish to mod my game. I plan to play through this game for the first time in co-op with 5 of my IRL buddies and if I use an mod they will have to use it as well and I don't think that they want to change anything in the game, they like me will most likely want to play through the game as it was intended, at least for a first run through the game.

I will keep what you said in mind for a second playthrough though.

It's not a mod, it's a game editor. Ever considered a Ranger>Cleric? (anyone who is paying attention will notice I mention them in every other post, lol)