I've made several of them over the years; I did my first Snow White when I was 16, for a high-school theatre production of a parody of the Disney version. (BTW, I later realized that Snow is the hardest of all to do; the stiff collar and slashed sleeves are a pita. There is rather a lot of piping, too.) My princess dresses are circulated through the family, some are already on their third child.
If this dress is to be worn by a child who will want to wear it frequently, or wear it at WDW, invest a little in the right fabric. You want something that will hold up to being laundered, feel comfortable for extended wear, yet look glamourous enough to please her. For a child, my recommendation is high-count cotton sateen, the sort that good bedsheets are made out of. It is rather hard to find, but the result will be worth the search. Whatever you do, stay away from rayon satin and/or rayon taffeta; they're cheap, but they cannot be laundered without losing color, and they get permanent crease-marks very easily.
Making a dress like this is an effort, especially if you are not used to making really constructed garments. To get the most wear out of it, sew it with large seam allowances and well-finished seams so that it can be let out as she grows. I recommend using hooks/eyes or loop buttons down the back, rather than a zipper, which does not adjust easily. It will be easier to adjust if you make the peplum detachable; hooking it into thread loops hidden in the seams will work nicely (velcro is good, too, but I like to stick with less visible fastenings.) Sequins are great if you want flash, but IMO, beaded satin cord along the seamlines looks nicer on this dress, and is less likely to disintegrate when washed.
Expect that making a dress will cost just as much as buying one, if not more. However, a home-made dress in a good fabric will wear much better and be a lot more comfortable than the off-the-rack nylon models.