Hi Steve f,
Thanks for your message.
If you just want to edit audio and use Cubasis on its own (no other 3rd party instruments and or effects), basically every current iPad model should be fine.
What audio edits do you plan to do in detail?
In addition, it might make sense to think about what you want to do with the iPad at all, and decide what model/storage makes sense for this.
Hope that helps!
Best wishes,
Lars