Interview them in person before you get them to come to your house. You can usually get an inexpensive flight from most European countries to the UK for a weekend.
Set the house rules before they arrive. No boys or friends in house without us meeting them first, never any overnight guests period, clean up after yourself, keep your room tidy at all times, etc...
Hours usually are around 25 for a normal Au Pair and 35 for a Au Pair Plus.
We did £80 spending money, mobile phone, monthly travel card for our Au Pairs. They didn't need to travel anywhere for the kids but it was a nice thing for them to have plus I always knew they could make it home. Phone was also good because I always wanted them to be safe and be able to call them at anytime while with the kids. We also had a list of foods they loved to ensure the house was stocked when they came and left really lovely bathroom stuff in their rooms to make them feel appreciated and welcome.
Communication is key. Keep things honest and open at all times and it will be a great relationship for everyone.