The best place to start is on the website of a few of the companies whom match au pairs with families - au pair in America, go au pair, cultural care. Go browse the faq on those sites and then come back.
Weekly rate is now just about 200. But you need to budget alot more than that - placement fees, education stipend, food, etc.
Kensington is not terrible but unless you live right close to a metro or major bus line you will need to give her vehicle access. I assume you expect her to get your son out to activities? We live two blocks from a metro but our au pair uses the car for pick up, drop off, library, play dates, and then some of her own socializing on weekends or evenings.