Above all else, an excellent temperament is paramount. You can have the best looking pony in the country but if it can't produce an immaculate individual show, then you will be propping up the end of the line (or, worse still, the child will simply refuse to get on board!)
The schoolmasters who are county standard will not be cheap. But they ARE worth their weight in gold, and generally hold their value pretty well.
Remember also, if you buy now, the pony will cost less. However, this should be weighed up against the cost of keeping a pony in work over the winter. Plus the associated behavioural problems that some experience when they do not get enough turn out/exercise etc.
Is this the child's first pony? If so, then the most important thing is that she has fun and gains confidence. Please don't be rushed into buying something.
I remember calling about a horse a few years ago that was for sale. I was told that I needed to hurry as "X" was coming to see it that very day. Which would have been fine, but I was the "X" they were referring to and I had not made any appointment to view at that point!
Pony Shopping is great fun - so take your time and the right pony will find you!