Yes, you need a registered Kindle, otherwise Amazon won't sale to you. The DRM is tied to a specific device. I don't think you can buy from Sony without a Sony reader either, for that matter.
As far as I know, there is no way to know up front if a book has DRM or not. Older books seem to be more likely to not have DRM. I guess you could download the sample of the book and see if that has DRM. I haven't tried that personally, but it's something to try.