Hi I personally have never employed one bit know a lot of people who have. Their main comments are:
Get someone with experience and not just a backpacker.
Get your kids prepared for the idea that the au pair may not want to hang out with them on their days off. This is the main reason we haven't used one is our kids are too young to understand that while the nanny is still in the house they aren't their nanny Saturday / Sunday etc.
Be very very clear about what you expect them to do all day. If all your children are at school is their day free for them to do as they please or do you expect them to do jobs and cleaning etc? The less you ask them to do the less you pay them. But be very clear. Friends of mine recently had a terrible experience because it was all a bit too casual and the mum felt let down whereas the au pair didn't realise exactly what she was meant to be doing.
$200 a week is pretty good for a live in. You may struggle to find someone for less. As for getting used to them in your house apparently everyone I know said they just slot in and it works.
Good luck x