Is it permissible for a girl who practices masturbation to get married, and is her marriage invalidated if she does so?

What you mentioned about the invalidity of the marriage of one who practiced masturbation has no basis in Islamic law. One who repents from a sin is like one who has no sin. You are not required to inform the suitor of anything. We advise you to constantly supplicate and humble yourself, to disregard these whisperings (waswas), and to consult a trustworthy psychiatrist.