Your "must-have" is rather "must-learn" at the moment. Try to learn how to use base figures, clothing, props, lights and so on (in other words, how to play with stuff that's already included in P6) and when you'll get a hang on this, you'll know what are things you're interested in - hence, what to obtain.

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Outdated gallery over at DeviantArt

Fics at FanFiction.net and Archive of Our Own (AO3)