I hear what you're saying, but the focal length (if around the 90-105mm) would be right and so would the magnification. You could overcome the depth of field issue that you understandably raise either by shooting on a tripod with a smaller aperture but a longer shutter speed or by focus stacking. I don't think you'll get the size of image you're after with a non-macro lens. (Well, you might with a large telephoto, but that would create other problems.)
that would depend on the ring flash that you choose. the canon one really only works with macro lenses and lenses about the same diameter. sigma's might fit on a larger diameter lens. take a look around and see what fits your needs.I like the idea of lighting though, would a ring flash work on anything other than a macro lens. Or would an image stabilised macro lens work?