Here's my ironclad rule on nannies: get one that is at minimum 50 years old and pleasantly plump. Not speaking English also helps.
Having a hot nanny is the worst possible thing you could have in your house. I am *not* speaking from personal experience in this, but I have witnessed several marriages break up due to a hot nanny/au pair/whathaveyou. In one case, there wasn't even an affair going on. The nanny developed a crush on the dad, he resisted her advances, the nanny kept at it, the wife got suspicious and then assumed the husband was cheating. In other cases, the husband did diddle the help.
But there's always going to be a certain amount of tension if you have a hot nanny. What happens if you assume you're home alone and emerge from the shower or bathroom naked and HOLY CRAP there's your nanny standing in the hallway, and then HOLY CRAP there's your wife just walking in the front door. If you have pleasantly plump 52-year-old Rosa Maria, then everyone has a good awkward laugh and gets on with their lives. If you have smoking fuckbox 19-year-old Oksana, then you've got a sticky situation on your hands.