I've been looking into the possibility of hiring an Au Pair for my kids (I'm likely to be a single dad, shortly). The agencies that I was looking at all had some pretty well laid-out expectations for Au Pairs and their living and working conditions, and their pay. The agencies all seem to charge a pretty exhorbitant 'fee' for their services, sometimes to both the Au Pair and the host family.
The agencies all had the same basic conditions for their Au Pairs, though. These included things like a private room, set working hours, days off, pay, and other expectations. The Au Pair was expected to provide their own health insurance, airfare, and cover some of their own expenses such as phone calls back home. (many of these things are able to be financed by the host family, as a draw against future earnings, but that takes negotiations)
Because the agency helps handle visas, contracts and background checks (which can be difficult in other countries), they seemed to be the best way to go.
As a parent, the longer term that child care can be depended on the happier we are. Summer contracts (May-September) would probably be the shortest acceptable to most American host families (to help keep the kids cared for during the summer vacation from school)
Au Pairs are often considered a low-cost alternative to a nanny, because the Au Pair is gaining something in addition to pay - a base of operations to travel and some spending money and time to do so! Because they're cheaper, it seems that people are all too often willing to take advantage of that. People also tend to forget that the 'base of operations' part is indeed a portion of the compensation for doing the job!
You definitely need to make sure you discuss everything prior to making the commitment to go out there. Since this is the modern age, things like finding the actual location of the place you'll be working at in relation to where you wish to visit, getting photos of the house/apartment/room you'll be staying in, and information about the family you'll be with shouldn't be too much to ask for. If you like children, it seems like a good possibility for travel, but there's a lot to watch out for, and it would definitely take some homework, or, at least, and exit strategy, in case things go badly...