Keeping in mind I've only done a few costumes myself...^^;
It shouldn't be too difficult to find a purple wig...depending on your budget, Amphigory.com or eBay might have something similar to what you're looking for...(upon looking at Amphigory, unfortunately it looks like the Cleopatra and Heroine wigs, which look like the closest style, don't come in that shade of purple. You could get the Enchantment in "Femme" and cut it...)
For the dress, you might be able to find a simple sewing pattern to use. For the boots, you might be able to make some sort of long boot covers out of spandex...the shoulder armor might take some creativity. You could possibly use some kind of foam kneepads or elbowpads and paint and decorate them to look similar...also, Amethyst Angel
has a nice tutorial for making armor out of foam...
Part of the fun of cosplay is experimenting, so you might want to try some things out, and then post if you have a specific question about something. ^_^ Good luck!