Kakashi says that it wouldn't be the "last time" they run into someone stronger than him
The obvious implication is that this was the first time it happened. It's pretty obviously intended to imply Haku is stronger
No, Zabuza is just more well known, Haku is a nobody in universe. Zabuza himself believes that Haku would be too much to deal with for a Kakashi who beat him.
Are you using Haku saying Naruto should kill him?
Kabuto should be stronger than anyone at the Chunin exams by a good amount, but I don't think he could do like 1v10 against Gaara and others(obviously I mean part 1 Kabuto)