For your situation, I don't thinking reading Manga is the best idea. You should focus on the language you will be using in the country. I suggest learning the most common vocabulary and Kanji (at least enough to be able to read signs and menus and the like). Also, think of situations you'll be in and learn how to make sentences for those situations (eating out, shopping, asking for directions, etc).
Focus on functioning.
You don't need to read novels and know 10000 words to go to Japan. Work on your speaking and listening and learn useful words and sentences.