Dress as you would to go to Mass anywhere else in the world. You'll find all sorts of people in the congregation from fellow tourists to the local ladies / families of the parish, some of whom are always dressed quite smartly even mid-week in mid-winter simply because that's the way they are. Re lunch, I would pre-book, if I were you. There are obviously dozens of places but Sunday lunchtime is always a popular occasion and Easter Sunday will see a lot of visitors in the city.