I don't think you overreacted, yes, it sounds like he finds you attractive and is hitting on you, but it doesn't sound like it was horribly offensive, nor was your response to him too harsh.
I think you could have responded to the "Do you mind?" with, "Actually, yes, I do.. I prefer to keep relationships at work professional." But I think what you said was perfectly okay, too.
I think answering his unsolicited chats with, "I'm sorry, did you need something?" would get the message across. And if he says anything else that you find inappropriate, a firmer "Please don't say things like that to me, I prefer to keep work relationships professional." Or a report to HR, if it's something more offensive.
I think that you are doing him a favor by pointing out that it's not appropriate. People can be really touchy. I work with someone who got reported to HR for smiling and saying, "Hello, ladies!" to a couple of co-workers in the hallway. And he's NOT the type to have said it suggestively.