Hi Copey - I have had a wonderful Canadian AuPair and can give you a bit of advice about hiring her "over the internet". Firstly, whilst she ticks all the boxes, you need to build up a relationship with her. I used to telephone her (2p a minute using call18866.co.uk) regularly and spoke to her family as well. After a month of these (and email) conversations I was sure she was the right one for us. But then I was looking for a unique person to become part of the family.
In terms of salary, I always had a question on my standard template as to what pocket money expectations they had. This always gave me a good overview of the person - ie "I have looked into it and it is around the range xxx" (shows they have initiative). I do know that the GBP to Canadian$ exchange rate is very favourable so this is another attraction of coming to the UK.
In terms of airfare - this is where I am completely different to others. I believe, for an AuPair, the agreement is that you pay for them to come over and they pay to go back (or the other way). But the way I viewed it was that I didn't have to pay agency fees so I paid her airfare (admittedly going home was a Christmas present). I was also very generous in terms of holiday. I used flyzoom.com and the flights were around £220 each way.
I didn't get a CRB check - don't know if such a thing exists in Canada but did follow up (using 18866) all of her references by phone.
One thing you may want to think about is her Visa - I know it took my AuPair around a month to get hers. Also, they are only able to come here on a Working Holiday visa - where they can work for a maximum on one year. Having said that, when my ap went back home to visit at Christmas, she got a letter from UK immigration offering her to extend her visa to 5 years (which she will probably be doing).
Our experience with our ap was a wonderful one and I would have no hesitation in hiring from Canada. But you may want to think about the time difference - they may find it lonely as they can't just chat to their friends over the internet in the evening. The way I got round this was to say to our ap that she could use 18866 anytime (and she did - sometimes over 2 hours) to call Canada. Okay our phone bill was £20 a month more but well worth it.
There is a lot to think about - if you have any further questions, please contact me.
By the way, we now have a Dutch ap who is just as wonderful. Have you had any applicants from Holland? Our ap is over-qualified (a School teacher) but just wanted to be part of our family and try and improve her english. I could ask her if she knows of anyone looking.