Robbie - I would echo Hedgehog's point about the English. Its the one thing I am really clear about with all au pairs (we have had three now). If they can't understand me and the children, and can't be understood by them, then nothing is going to work. So I only have au pairs with good English. As for your other questions - well, you do have to get used to someone else being in your home, but we have a policy that the sitting room is for me and DH after 9pm - the au pair can use the playroom (which has a computer) or the big kitchen, or her own room, which has a TV. You need to know you can talk about things or have a row in privacy!. About the phone bill, our au pairs pay their share. Each month I given them the itemised phone bill and they mark their calls, add it up and pay me. Obviously it depends on trust but I have never had a problem.
Food costs - I reckon about 10-15 a week. They buy their own toiletries etc.
Loneliness - this is where the au pair agencies earn their money, I think. Ours gives the au pair a complete list of other au pairs in the area, and basically all the girls then hit the phones and arrange to meet up. We usually offer to take her somewhere if we are going out eg to the cinema, but they tend to want to do their own thing.
The only thing you haven't mentioned but I would think hard about is the age of your children, if you are expecting the au pair to be in sole charge. We made the shift from nannies to au pairs when our youngest was 4 and just starting school - in retrospect this was too young. The children need to have some sense of their own routine/responsibilities, - you can't expect the au pair to remember that the child took a coat to school that day and that they need to bring it home, or that tomorrow they need to take old jam jars to paint in class, or whatever. I should have waited until youngest was 5 or 6, I think
Now the boys are 9 and 6, enjoy having the au pair around, learning a few words of Italian and how to prepare great pasta!!
Hope this helps