"He is a man."
When you're saying Ö férfi, you're already saying that he exists. And van is like "there is".
I'm recently learning Hungarian just like you. As it is an Ural language in the roots, I guess "Ö férfi" may direct the man like "this man" and you already mention "he is a man". Is it a close persumption?
Well, Van means "is" (in terms of "there is") and egy means "a /one" (there's no difference between the number and the article.)