Are there restaurants open for lunch or places where we could buy food?

Yes

Van Gogh cafe and Rat's Restaurant are both open. Please check the website for hours.

Van Gogh can be accessed from the grounds; Rats must be accessed through a separate entrance.

There is only one place open. Rats restaurant you have to make a separate reservation.

They have 2 dining venues on the grounds. One is very expensive. Check their website.