For my daughter just beginning with a pony club, I am nervous to go out and buy too much stuff in case she decides in another month that she no longer wants to be involved. It's too much money to shell out for nothing.

So what do I need right now? Is there anywhere you can rent stuff from? What about a saddle, does she has to have her own?

I think it would be best, if you consider her age, you will want a saddle that fits her proper. Remember, everyone is shaped different, just like each horse. Maybe it is possible you can get one that will fit you and her both. :)