Well first you need to narrow down what you need from your childcare.
Before/after school? School run? By car? Going to activities in the evening? Homework supervision/help? Do you need a bit of help around the house? What about holidays?
A city is good because there are likely to be other au pairs, public transport is reasonable so a car isn't a necessity and language courses should be readily available.
Many people in your situation find that an au pair works well but your DD is getting to an age where she may resent being left in the care of a teenager, therefore the age of the au pair might be a consideration. An au pair will typically do school runs and after school supervision, and depending on their level of English/general education may be able to help with homework. Some can cook, some can't. Some are willing to pitch in with household chores, some aren't and do the bare minimum.
There are pros and cons to long and short term au pairs. If you need them to be sole charge then a week at a time of handover/settling in is quite intensive every 6 months. On the other hand it's less time for them to start to grate on your. I wouldn't regard children not getting attached as a good thing, personally.
You do need to have a good contract and make your expectations clear. This goes both ways, if you say your au pair can take language classes then you need to regard the time they spend at college as sacred and find other cover for inset days/if your DD is ill. Many young people are expecting a part of the family deal - if that's not you then you need to be extra clear about that.
You can go through an agency (but make sure you carry out all the relevant checks yourself too), although they may not be very helpful if it doesn't work out, which for many people is the principle benefit of using an agency. You can recruit via au pair world or great au pair, both of which have a feature where you can subscribe and contact candidates for much less than an agency fee. Some pointers for self recruitment:
write a very clear ad
have a checklist -for example driver, EU nationality unless you want to extend it to Aus, NZ, Canadian or Japanese candidates with a Youth Mobility visa (I would also avoid Romanian or Bulgarian au pairs as they aren't allowed to work FT e.g. in school hols), language (e.g. your DD is/will be learning Spanish and you don't speak any at all), level of English, level of education, previous experience, hobbies
measure every candidate against the checklist - if they don't fit then move on
if they do fit then message them and start talking
interview thoroughly via skype to get a good feel for their level of English
check references thoroughly
consider flying your ideal candidate over for a trial weekend - easyjet are still cheaper than an agency fee
Have a read on here to find out more by searching threads about au pairs. Bear in mind that people often post when they have a problem so what you read isn't necessarily representative of the experience.