You don't need stereo microscope, you are inspecting a surface (or even a line) and don't need to see 3D, but it's probably nicer to have if you do.
The magnification is not the critical issue, it's the resolution, i.e. what level of details you can see. A high quality optics will show you a lot more detail at 30x magnification than low quality at 100x. Yes, it would be 3x smaller picture but there is no value in a blur no matter the magnification.
The one I use is Leica from the 1930s and looks like the one on the right in the picture in post #2. It depends what you want to see. With mine at 60x magnification I can tell the difference that a 8000 grit hone makes. I am not talking about the scratches, but the roughness of the edge (teeth if you like, even though they are not teeth).
So, I can hone a razor completely by simply looking in the microscope and not testing the edge in any other way.
If I couldn't see that level of detail, for example if I used a loupe or the radioshack microscope, I would use it while setting the bevel to make sure there are no microchips left along the edge (I could do that even without magnification as long as there is strong light), and afterwards I'd use other tests like how the razor interacts with the hone (tactile and visual, may be even auditory on some razors), how it interacts with hair, or TPT to determine whether it is time to switch hones.
A microscope is just a tool, it is not necessary for honing, there are other tools which can accomplish the task of telling you when to switch hones. Sometimes though if things keep failing for unknown reason a microscope can be very helpful in determining the problem.
As any tool you have to learn to use it correctly though. Depending on how the light falls on the blade you may not be able to see any detail, but that's easy to fix - you just move the light or the blade around until you get the correct exposure. Once you figure it out it's pretty easy to inspect the edge quickly.