I'm an undergrad to grad person (PhD no MA). And I will say this:
The people who come in with an MA get fewer years of funding and need to start with a dissertation topic out of pure necessity and tend to change fields less frequently. You, however, would get the luxury of an extra year to account for coursework. A lot of people in my department have found themselves changing fields from Latin America to Europe, etc. So it's not uncommon to put something down and change your mind. In fact, it's kind of expected and by the time you start no one in the department remembers what it is that you said you do anyway.
With that in mind, I would say organize yourself around what you know and what you've done. Give them a clear picture of where you fall generally (15th century France) and based on work you've done, where you think you'll go. You don't need to go around reading tons of books to know where you might fall in the field or what is currently going on. Get out some journals for your field and peruse the book reviews and articles to see what's getting reviewed (attention) and what isn't. The book review is your friend. Try H-Net reviews also and if you can find syllabi for the reading survey courses in the departments you're looking at that'll give you a good idea of what they think is important in the field (and often they pick really current works that are groundbreaking in some way) that everyone should know.
Self doubt and insecurity appear to be a common trait amongst all us academic types, but I think this is a good opportunity for you to decide that maybe you can push forward even though others are doubtful. It's definitely a valuable lesson. It's a tough one, though, and never forget: there's always next year.