You need to be prepared both the case portion of the interview and the fit/behavioral portion of the interview. The case portion takes longer and requires a longer runway, but don't underestimate the fit - it's evaluated just as heavily!
I recommend the following for you:
1) An initial planning session with a coach: 1 hour with a coach now will have a productivity multiplier effect on all your efforts moving forward. They will figure out what materials are best for you, guide you towards the best ways to learn, and come up with a preparation plan with you.
2) Leverage free resources first: PrepLounge Q&A and case library, Poets and Quants, SpencerTom, Google, etc.). Leverage these options, read-up, and over time you'll get a feel for what you really need and where you really need to invest your hard-earned $
3) Case with other PrepLoungers: Casing with other PrepLoungers is free. Not only do you get to practice casing, but you get direct feedback. Additionally, you learn a lot just from casing others. Finally, from other PrepLoungers you'll learn which materials/coaches are helpful.
In summary, while free options don't beat paid options, you can use them for a while to get a feel for what works for you. Have an initial coaching session to get you on the right track, then go the paid route when it's clear either 1) You are stuck or 2) It's clear the paid route will improve your productivity/progress