In all honesty mate, if I were in your situation, I would at least wait until she was 16. I can understand a lot of people disagreeing with me and not understanding, but from my previous experiences I'm really uncomfortable with age differences. About 4 years ago when I was 14 and my girlfriend was 13 (we were in the same year) she ended up running off with a 19 year old loser and didn't stop bragging about having sex with him. It made me feel sick that someone who was 19 didn't appreciate that she was only 13, that's ONE YEAR OLDER THAN TWELVE.
Although in your case, being 15 she's much more mature than the average 13 year old girl. I think your idea of speaking to her father may just make things harder if you guys did go out. That way her dad would know you had second thoughts about it, considering wether being with her was wrong or not and had to decide.
You may completely ignore/disagree with me and I understand that, but personally I think waiting a little while and getting to know her a bit better would be for the best :o