They are a good resource, but I have no personal experience with level 0. I bought my first ones when I was around JLPT N5 and went directly to level 2. I would be inclined to push up a bit higher with these and have to order a lower level, then to order the low levels and have it too easy.
If you have your sights set on taking the JLPT exams, these are great for listening practice since there is not really much material out there that are real comprehensible stories for people at very low levels. Yes, the packs of 5 books are expensive, but factoring in the CDs makes it easier to swallow.
If you are in USA, your best best is to order via Amazon Japan. I bought 2 packs during a trip to Tokyo - the remainder I ordered a few at a time through Amazon around a few Christmas times. I’m thinking you might not get a break on shipping if you order multiple packs, so it makes sense to split into a few smaller orders over time. Play with the order page and see.
Once you get through all of these, do check out the Magic Tree House books in Japanese. Those books have TONS of material per book to get through and the grammar is all really easy - I think JLPT N4 level will get you through them, but the expanse of vocabulary needed is much wider.
Funny story with katakana… I learned this LONG ago because it meant I could look at menus and figure out what to order without knowing ANY Japanese. I learned it first because of that… then fast forward many trips later. I went to an all you can drink bar where the whole menu was nothing but katakana and by about drink 6 my ability to decipher it was greatly diminished to the point where I thought I had forgotten what I learned first! Yes, katakana is challenging since it is trying to make an unnatural fit to works that weren’t meant for Japanese pronunciation! Certain words will jump out easier with repetition, but new ones will never read easily like words in hiragana. Oddly, once you start to know a decent amount of kanji, you’ll be annoyed by words spelled out like 友だち instead of 友達.