I always thought, it meant "Welcome" (?) Am I then totally wrong?
You're not wrong. MarHaban can be used as both 'welcome' and 'hello'.
Does this word have any specific meaning?
What do you mean by specific? It simply means Hi, But if your asking for that root it would be long story (root is from ترحيب welcoming)
What about assalamu alaykum?
It's very common, some say it both (Hi, Asalamu alaycom)
Does 'Ehlan' mean the same?