This new feature may be helpful for you to look at too: www.aupairworld.com/en/blog/aupairworld-family-handbook
I have had great success with APW, but I find it's better to recruit as early as possible. Also, be open to shorter placements.
Note that if you are a UK family, technically you can only take girls from the EU and a very limited number of other countries. That does not include America (unless they hold a British or EU passport): www.aupairworld.com/en/au_pair_program/uk/family/regulations
Keep a spreadsheet / notes when you do Skype interviews as you will get confused between the girls - believe me!!
Also, if it is important to you that the girl stays for the full time period, you could say you will pay a bonus at the end if they complete the full year (or however long you are asking them to stay).
In the UK, there is no requirement for a contract and there is no government approved standard form contract. There is an EU one (available to download from APW), but lots of it is irrelevant, so you would have to score a lot of clauses out. I am a solicitor and have never bothered with having a contract because it's not really enforceable anyway. It does help to have a document that sets out hours and all important terms though so there are no misunderstandings.
If your girl is from the EU, make sure she gets the equivalent of an EHIC card to entitle her to free health care here. With a Tier 5 Youth Mobility visa (if she is from Australia), I think they have health insurance included or it makes use of the reciprocal health care arrangement between the UK/Aus (I can't remember exactly.