MB with twins.
Re food - what you provide or not is up to you. We stated in our contract that we will not provide food for the nanny but he/she is free to bring whatever she likes and we provide cabinet space for her to keep food at our place. (We also provide some space for toiletries and a change of clothing, etc...) As the relationship has evolved it frequently happens that I cook food so that there are leftovers for the kids, ample for the nanny to have if she wishes, I'll sometimes buy things I know she enjoys when I do the shopping, she'll sometimes make a big meal and leave dinner for us, etc... But it's good to start off with some clarity and I preferred not to add another person's needs to my routine shopping.
Chaos - get used to it.

Any experienced nanny has probably seen a wide range of things in people's home. Your sensitivity to keeping things under control is great - and hopefully you'll find the nanny to be an asset in managing that as well. (If she keeps order to all the kid areas during the day it will be easier for you to maintain that the rest of the time.) Try never to leave your messes for her and try to let her come into a reasonably calm, ordered environment at the start of her workday. If you picked the right nanny for your family you and she will become a team in managing your kids' environment and needs.
If your nanny has twin experience that will be a huge help to you also.
Good luck and congrats on the twins!! (Have you joined your local parents of multiples group yet? I have found that network to be incredibly helpful, especially in the first couple of years.)