I think that it is best to work out the specifications for your particular job and then decide who is best suited to it.
As others have said, au pairs are not meant to spend long hours looking after young children. Even if they agree to it up front, they are going to very quickly realise that it is a bum deal for them. The main reason for becoming an au pair is to learn English and to do something exciting for a few months.
I have a very traditional au pair set-up, in that I am a SAHM and use the au pair for light cleaning, school pickups, playing with the children and babysitting.
I think an au pair can work for a WM if the children are all at school. Then the au pair would be working from, say, 8 - 10 am and 3 - 6 pm, and would have the middle part of the day free for language classes and anything else she wants to do. You could also consider an "au pair plus" to work 35 hours a week (EU and Commonwealth countries only) - but she would still need a chunk of time in the day free, so this would work out for, say, 7 - 10am and 3 - 7pm.
As for money, £50 is low. Even if the girl says she will accept this, she will soon get disillusioned when she sees how expensive things are here, and when she makes friends with other au pairs. I find that it is best to give her all her pay in cash and not rely on little tips, treats and bonusses. I pay my au pair £70; I will get an au pair plus from September, paying her £100.
The other thing to keep in mind for an au pair is that they are very mobile, and I think that it is a rarity for any to stay for a year or more - especially if their English is too good for free language classes and they don't want to fork out £300 for the paid ones. I actually prefer short-term placements - basically working around the school terms - because by the time they start getting fed-up with your domestic scene, it's almost time to go, but it does mean more work sifting through the applications. I would feel nervous using an au pair for childcare while you are at work, unless you have access to good emergency childcare or a very nice employer.