"I am not looking for a man."
Translation:Én férfit nem keresek.
It sounds like you are talking about a certain man who you don't name. "As for a certain man, I'm not looking for him. (For the rest - who knows, maybe)"
You could say "Egy férfit sem keresek" (lit. "not even one man") if you want to say you're not looking for men at all.
(The "certain one" feel is achieved by word order - you put it at the beginning of the sentence where you define the topic of the sentence. Preferrably, the topic is something but participants know and therefore change of information is possible. This pretty much contradicts the nature of talking about a random instance which you should somehow introduce first.)
I hear a difference in meaning between the two, although they are both valid translations of the English sentence.
Férfit nem keresek means I'm not looking for a man (end of story, I'm not particularly looking for anything else either.)
But Nem egy férfit keresek would be compatible with going on to say that you are looking for something...
Nem egy férfit keresek, hanem egy nőt - I'm not looking for a man, but for a woman.
Or even, Nem egy férfit keresek, hanem húszat - I'm not looking for one man, but twenty.
It suggests this so strongly, actually, that if you said to me, Nem egy férfit keresek then I would probably reply Akkor mit keresel? (Then, what are you looking for?)