Saving up for 6300 takes along time buddy. Unless there is a champ that you know you'd love playing, I'd say just buy 3-4 cheaper ones.
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I suggest you take advantage of the free rotation. It took me a while to realize it but its all benefits to play each champion at least once. It allows you to get a feel on different playstyles and you might find an interest for a champion you didn't think you'd like. Like people said, once you spend it, its gone, so make sure it goes to someone you already know you can play while enjoying it.

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Later on, when you have mastered some of them, go and get the rest. But only get expensive champs (4800ip+) if you are sure you want them.
I, for example wanted to get Renekton but tried him in free week and thought he sucks, so I got myself Blitzcrank instead.