The laws of optics are agaisnt us when it comes to depth of field
with high magnification. Unless you're willing to experiment a lot
with calculating reciprocity failure with color emulsions whether print
or slide, you're going to be stuck with very shallow DOF. Really long
exposures are the only way to work with very small apertures, and reciprocity
failure causes truly bizarre results with color films--hideous colors, etc.
If you go much smaller than f/64, diffraction problems ruin the sharpness
of the image. I've seen view camera lenses with stops down to f/128, but
unless you're a real genius with a camera, I don't know who would use such
small apertures.
The ring flash is a good idea--pricey though.
J. del Col