Well, I would suggest buying a new wig that is her color hair. But noone is stopping you from using the one you already own.
For the costume I would suggest buying a pattern for somthing like an alice in wonderland dress (or something similar) and alter the sleeves to make them longer. And maybe the hem of the dress depending on how long the one for the pattern is. The cuffs are tricky but its basicly a strip of white fabric with a stiff interfacing between the two layers of fabric to make it stand up like that.
The apron is pretty simple in the front buy detailed in the back. In the back it is just laced up. So i guess think like its a apair of sneakers and try to construct it that way. The bow on the back can be easily made if it detachable and you can attach it with something like a saftey pin. I know of a good tutorial for the bow and ill give you the link as soon as I can.
I plan to do this costume myself and what I just told you is how I plan to make it.
Sorry if this isnt of much help I am fairly new at sewing myself. And if you have any others questions feel free to ask.
edit: here is the bow tutorial- http://browse.deviantart.com/?qh=&se...orial#/d2hwrzt
i have used it before and its easy to follow and the bows turn out great!