Is there too much of an age difference to date her?

I think an age difference much bigger than you have now would be problematic, but you should be fine. That's not a difference that can't be overcome. The fact that you say you don't plan on taking her virginity is a good sign. A lot of older guy/younger girl relationships run into trouble for exactly that reason.

If she knows about your partying past and you're willing to give it up for her (or already have given it up, which is kind of what this sounds like), then I would definitely go for it! She sounds like a fantastic girl. And it sounds like she's got a good influence on you. In my experience, if a guy says that a girl deserves someone better than him, he's usually ashamed of something he's done in the past, not something he's still doing. And usually, he's a better guy than he gives himself credit for.

Maybe this is divine intervention to change your partying ways! :P
You're 19 and she is 17 and she is too young for you? That sounds about the perfect age. Is not like you're 19 and she is 15.
About asking her out----well, are you planning on going back to your 'partying' ways? Is not like you're 26 and getting older and bored of the party scene. You're at the prime age when the partying is only starting. If you honestly can say that you're done with that part of your life, then ask her out. However, if you think you will ask her out, and then screw up, then maybe just tell her that you would love to go out with her, but you're not ready to give up your partying ways. Anyway, it seems to me you really want to go out with her, but you're just afraid of losing your partying ways, and thus, you're coming up with excuses not to ask her out. :)