I think the reason why Ben didn't tell you about the baby for so long was because he was ashamed of what he'd done - broke up with the girl and left her with a baby (even if by accident). I think that he didn't want you to think badly of him, and he seems to like you a lot and he was probably scared that he'd frighten you off.
Yeah, if he is going to be involved with the baby, then he will have some kind of contact with the mother for 18 years, but that won't change their relationship. A baby cannot, and will not mend or alter a broken relationship. And nobody should try and ever prevent a father/child relationship.
And yes, the child should come first, always. However, that won't stop the father having a life of his own or being a good partner, and quite frankly, if you are still interested in Ben, then I would say just give it a go. Think about what all this says about him as a person - he didn't want a baby, but he's willing to stand by an ex, who is having his child. He sounds like a decent guy - how many guys out there do you think would stand by this girl and this baby in a simular situation?
Besides, if you don't give it a go with his, will you always wonder, and later regret it? Possibly. If it doesn't work out, you can always break up, but you might not be able to get him back if you say no now, and he finds someone else. If you're really not sure, then just take it slowly and don't rush things.
At the very least, I think you should remain good friends with him, and be supportive of his situation, because he's going to need his friends now.
Good luck with whatever you decide to do.