What 3x3 would you recommend I use? I was thinking C4U but I'm honestly not sure if that'll work.
I used Rubik's brand 3x3x3's to make mine just because I could buy them easily. You should pick a cube similar to a Rubik's brand because it will be easier to cut than something like a dayan guhong. I don't know what the internals of a c4u 3x3 look like, but if they are similar to a Rubik's they will work.
Which pieces do I use from the 3x3, only edges?
I used all of them. I glued edges on inner circle edges and corners on inner circle corners. This make the outside look nicer.
What type of glue do you recommend? Just super glue, or something like pc7?
I used normal super glue. If your rough with the cube eventually a corner may pop off but it can easily be glued back on.
I assume your instructions will make a bit more sense and be easier to visualize once I've got the puzzles in front of me yes?
Yea, once you have the puzzles it will make a lot more sense.