of course they can get drity, not because they;re white, but because it right on the ground and next to the brakes.
Do you have a picture of your stock wheels? some oem wheels looks nice which if you like enough you'll never have to go aftermarket.
Worth buying? that's really all up to you, i would put white wheels on my car, i see why not.
any wheel can get chipped, that really all depends on what type of paint/clear coat is on them.
if your looking for just any wheels i would start off by doing google searches. If you know the make and model of a wheel that your interested in, look for resellers, or contact the manufacture directly.