Get a good domain ( old one ) and put unique and quality content and apply for adsense. This is the best solution. Blogspot always have some problem or other.
A domain is a good thing to have but I don't think you have to buy one just to get approved for adsense. The message they sent you was "innapropriate site content" so that is obviously the problem. Someone suggested you rewrite the content until it is original. Outside of that, I wouldn't bother with too much else.