The reason I am asking this is that this sounds like a very popular scam, and what this so-called father is trying to do is to blackmail you. I find it very difficult to believe that a police sergeant would countenance blackmail. And why on earth should you have to pay for his daughter's wrecked car?
This seems like a sextortion fraud. Here's how that works:
Unscrupulous people hover around on chat rooms and dating sites, and they lure in their victims by assuring them that they are of age but then trying to engage them in sexual chat and/or pictures. Then, at some point later, the target of the scam gets a phone call from a supposedly angry authority figure. But what that so called authority figure wants is money or he claims he'll prosecute.
That's blackmail, of course, and one should never give into that.
You did nothing wrong. There is nothing unlawful about having a relationship with a minor so long as you are not having sex. If you have proof that she said she was 18 at first and proof that you repeatedly told her you had no sexual interest in her at all, you did nothing unlawful.
You should have nothing further to do with this girl and her so called father. You should find yourself a local criminal lawyer, just to be on the safe side. You should not be talking to the police but the lawyer can determine whether the sergeant is for real and actually made contact with you. In that way you won't incriminate yourself further.