I'd definately take an over night bag.
As for nappies I was told when DD recently had surgery to put her in what ever she wears at night, we used a pull up rather than a nappy.
Pain relief it's up to you if you can easily access a chemist otherwise grab some pain stop night.
I'd also take a portable DVD player or tablet for entertainment (one places supply DVD players)
She might sleep heaps in recovery especially if she's had morphine, or be like my DD and not :/

Good luck darl, she will be ok

Xxxxxx

ETA its very normal to feel anxious, you also need to decide who goes in with her, maybe send DH.
DD1 is having surgery next Tuesday, so I feel for you