Hi Dug,
your choice of suppliers to buy from sounds good to me. If you buy from one of these suppliers, you can be sure that you'll get something good.
The tonal range of djembes varies widely. Very high-quality genuine shells that look identical often sound different, even when fitted with skins of similar thickness and origin. Each shell seems to impart its own character to the sound. The type of wood that is used also makes a difference. And I suspect that even with the same wood, there are still changes from tree to tree, depending on its place of origin, whether it grew with more or less sun and rain, etc. These factors influence wood density and surface texture, and so affect the sound.
Even the sound of the same shell can change dramatically when fitted with a different skin. From my experience, having a good shell is a necessary prerequisite to getting good sound: if you have a shell that is bad, it will never sound good, no matter what skin you fit.
But a good shell isn't a sufficient prerequisite: if you have a shell that is good, it can still sound really mediocre with a particular skin. (This has happened to me more than once. Occasionally, after fitting a new skin to one of my good shells, I end up with something that sounds anaemic and won't come good, no matter how I tune it. Then, with the next skin, the same shell sounds awesome again.)
Skin thickness has major impact on sound, but there is no rule to say that "thick is better than thin" or vice versa. They just sound different, and which one you consider "better" depends on what you like and what you want to use the drum for.
As a rule of thumb, thin skins sound brighter, with more overtone-rich slaps. Thick skins tend to sound warmer, with slaps that sound more like very high-pitched tones instead of screaming slaps with a big overtone spectrum. But either kind can be an awesome solo drum, and either kind can be very loud. You can get great sound with
really thick skins. An extreme example are the Burkina Faso djembes that are strung up with calf or cow skin. (You can hear a great example of this on Baba Touré's recording "Daakan".) And thin skins can sound awesome too. (Listen to some of Bangouraké's recordings for an example.)
I don't know of any web sites that offer sound samples that compare different shell shapes and skin thicknesses. I'm not sure how much good this would really do anyway--there is so much variation in sound among shells that look identical that such a comparison would probably be meaningless. And sound samples, no matter how well recorded, have a hard time doing justice to the sound of a djembe. Most people don't have a stereo system that can come even close to accurately reproducing the sound. The sharp transients in the sound are very challenging for electronics and particularly speakers. And, in my opinion, no recording can really capture the full impact of the transients or the subtlety of tonal range that you experience when you hear the same instrument live.
I would suggest that, if at all possible, you make your way to one of your chosen suppliers so you can compare the sound of different drums in person, and pick the one you like best. If you cannot do this, I would describe the sound that you want as best as you can to the supplier and let them pick a drum for you.
Over time, you can experiment with different types of skin to see what you prefer. And, as you play and get deeper into the music, chances are that you will end up with more than one drum anyway. So then, you pick up the drum that sounds most like what you feel like hearing right now

It's a long journey, and for most players, there are many drums along its path. There is no such thing as "the one drum"--there is only "the first drum"

It takes patience and years of playing to develop a feel for what sound you like, what kind of drum and skin make that sound, and, as time goes by, your personal preferences will change, so the "best" drum from five years ago may no longer be the "best" drum today.
But you have to start somewhere. So, if you can't pick out a drum in person, trust one of the suppliers you have chosen, You will get something nice from any of them, and you will have started your journey.
Good luck!
Michi.