If you're looking simply for more spell space download an action bar addon like bartender4 or dominos. Most UI takeovers will be extremely different than the blizzard UI but in that respect I do and would use elvui
Can't ever go wrong with the default. You can always make modifier macros for your abilities for more barspaces. I only use 3 bars and have 123456qwrtyaghzxcvb with shift, ctrl, and alt modifiers. That should probably be enough keys
You don't need a special UI or anything. I do find, however, that ElvUI rocks for arena at least (gives me comp before we even enter). The default UI will work just fine. I only recommend taking something for rune/disease tracking like Rune Tracker or similar.