I need some advice on how to properly play a thief/assasin.
I know I should use light armor, but what about daggers vs swords?
Is it possible to avoid becoming dragonborn?

The thiefs guild and dark brotherhood are appropriate quest lines, but wouldnt the other quests take the feeling of being an assassin away?

Im currently using a mod which increases difficulty (percentage damage taken/dealt) but I think that with being an assassin you should quickly kill targets off? Currently a powerattack from a 2h bandit takes 50% or even more health, while mines do like 15%. And that is surviving with taking in account that I can block some attacks.

If you don't want to be Dragonborn, there are alternate start mods, and also a mod that's called "non dragonborn with dragons".
You can also stop doing the main quest at Bleak Falls Barrow. Like, say, your character thinks Farengar is an arrogant #$*¤￡ and don't want to do some fetch quest for him. That means you'll get no dragons, but it's the best way to avoid being Dragonborn in vanilla.