I would like to know for certain that I am the father before I develope a relationship with this child. But I also don't want to disrupt her life.
At this point, kiddo is 18, or soon will be. If you do manage to make contact with her, it would be up to her to consent to or decline a DNA test.

The time to act, unfortunately, was when your ex girlfriend told you she was pregnant, by staying in contact and establishing paternity at the appropriate time. I'm afraid you're too late, and will now have to wait for the girl to make contact with you.