not much experience in wigs unfort, i normally just buy whatever i can find on amazon. they tend to be cheap at least! i'd say if you struggle to find the wig of the specific character you want (or its expensive) you can always google "white haired anime characters" and see if you can find a cheaper one of a character with a similar haircut!
as for clothes, my real expertise:
i wouldn't bother with buying shoes ESPECIALLY if you plan on taking her to cons. you want to be comfortable and shoes are expensive!!! i personally have plain black boots that go with almost every cosplay i own, but sneakers are always a good choice too. no one will get on you for the wrong shoes. lol.
tights are easy, you can buy a cheap pack of them at even walmart probably. def any clothes store
the dress: if you want to make it yourself there's plenty of free patterns online like moodfabrics. adjusting patterns you find is also an option! sewcity sells plenty of great patterns for cheap too, AND you can often find patterns for sale at thrift stores or crafts places. check out your local library as well if you still want free
picking the cloth is super important imo, fabric can run high prices but you'll thank yourself for picking higher quality thicker fabrics than thin ones that don't drape your body properly.
for the jacket, honestly i might get a lab/doctors coat, cut our the back, and use that extra fabric to adjust the puffer sleeves, lol. this looks like a fun project!! keep us updated :D