I would say someone from fruits basket, maybe the yankee or esp girl if not just go kunoichi lol. Ritsu off of K-ON isnt bad either wit your hair. Good Luck at your first convention (^-^)
I am pretty sure a full Hungary dress is sold on Jlist, and a ton of other online stores. From what I've seen of people using it in person, it seems good quality.
Just not sure about the variety of sizes available....mostly medium.
Of course you'd have to get the accessories and shoes yourself, but that's the easy part. :)
You know if it were me, I'd just grab any school uniform from any anime you please and just go as a random unknown. You can represent your favorite show and not have to worry too much about anything else!
Spoiler:
For a moment, I toyed with the idea of you cosplaying as Mari Illstrious Makinami, but you only look normal happy, not insane happy