This is just my opinion, but from your post & the title, I would say you not only don't need anything else, you should start with 1 thing at a time. It seems like you're overwhelmed.
What I mean is, hook up the MPC, turntable & mixer & just use that to start. When you feel ready to add something else, like bass, hook it up & add bass. Ect, ect, ect. Even if it's the same day, just go by how you feel, you don't need everything all at once hooked up, right now, to make music.
If you do 1 thing at a time........& just DO IT....you will learn. You will KNOW what you need. When you come up against a roadblock, like how do I midi my synth or how do I sample my bass, you can work on 1 issue at a time, which is way easier to handle then everything at once. You can do research & easily find the answer. You will have a specific question that people can help you with.
Before you know it, you will have everything the way you want it & ready to go
In all honesty reading your post I can't figure out what serious help you need ASAP. I'm just saying.....not trying to be a b1tch..... just trying to help......