So far that looks pretty good. Only part I would consider changing is the motherboard for a better brand such as Gigabyte or ASUS - AsRock are a lower end brand yet aren't that much cheaper than a much better one.
There is no real point buying parts at different times, I'd just wait and get them all at once. That way you won't have any issues with return periods or wasting part of your warranty while you order the rest of the parts. Also, if you are buying multiple parts from one retailer you will usually save on the postage as this will combine with ordering multiple items at once.