A mamin, I thought the minhag of wearing white was because the day of the chassanah is like Yom Kippur for the chosson and kallah, and just like a man wears a white kittel on Yom Kippur as a symbol of purity, since all of their aveiros are being forgiven that day, the kallah wears a white dress. I do not know what the chassidim do. I do know that a lot of kallahs are not wearing pure white but rather cream these days.
As far as luxury of a kallah dress, most people I know take one from a gemach, pay a minimal fee for upkeep and return it in the days after the chassanah for the next girl to use it. At least 3 or 4 girls can wear a single dress (obviously not all at one time) depending on how many alterations are needed for each girl.
I was told that the dresses are fancy so that a poor kallah can have “jewelry” on her dress like a “rich” kallah. Of course we all know that girls still wear their jewelry to the chassanah, so somewhere this idea got diluted or forgotten. I’m just relaying what I heard many years ago.