Don't you think a free version is needed to so you can try it out before buying? It's my impression that most people want to be able to try out the functionality before buying. But limiting the free version some more or adding ads and having a cheap ($1-2) full hiragana and katakana version may be the way to go.
I agree, you definitely want to keep some sort of free tier. There are a lot of apps out there that claim to do this, or something similar. After trying a bunch of them I found one or two that I liked, but for the most part they weren't very good. I would not have wanted to pay to find that out.
I think you wouldn't want to offer less that one full alphabet, so people can get a good idea of what it will be like to learn using your app. If it's just a few characters that they can practice with, then they won't spend enough time with your app to make them feel like they should purchase it over trying another one out.
