Yes you can have an au pair in quarters you must tell the DHE. Hill sea is ok as are the schools. Accommodation is in short supply and as you have a house?? You will have to justify why your moving in.
This sounds odd but it's good your in private rented and the landlord is selling. As in the choice is not your so to speak. It not that bad living on the part I did it for about 15 years. Let us know how you get on.