Is it correct to say that there isn't much difference between a dual a quad-channel kit for RAM other than the vendor has matched them up?
Correct. No difference at all.
If I upgrade to 16GB of RAM for some reason, then I could run two dual-channel kits just fine. Are factory-matched kits only useful for when running max RAM or running at a high clock speed?
There was a time when it was problematic to run different brands and so on. In fact, motherboard manufacturers produce lists of acceptable RAM to be used with their product. You can find the memory compatibility lists at the manufacturer's website. Mostly this is a resolved problem, but I still recommend picking directly from the manufacturer approved list of RAM. It's easier. Factory matched is better for everything; stability is key.
Ideally you only run things from the same vendor and speed at the same time. Even more ideally you get units with the same model number; Corsair will be able to supply this if you want to buy subsequently. You can search by the model number at Newegg and Amazon. For the sake of simplicity it's easier to buy it all at once if that suits you now.
What is a good brand? I want to go with Corsair. While their best RAM is pricey, it's high quality.
Corsair is good. Crucial is also good.