So many questions! - A GOOD thing!
A purely natural sequence is certainly 'doable'. You broadly group naturals into coarse medium and fine stones - arato, nakado, and awasedo.
So for coarser stones consider Hirashima - very coarse, Kyushu Ohmura (NOT synthetic), Amakusa less coarse are options MArk or I have
For Nakado consider Binsui and Igarashi. Also Tajima and Kouzuke. Although very formally speaking aoto are not in this hieracrchy, consider them as a middle grit stone too. There are various Aoto and in this next incoming group of stone, expect this category to increase a good bit. I consider aoto as a must have category.
Polishing stones - the Hakka Tomae, Ozuku Asagi, Yaginoshima Asagi
Not an exclusive list above by any stretch, but a good start. And this is a really quick overview.
Great ones? Well price doesn't tell the whole story at all. There are bargains to be had, yet still meeting your requirements. And some very unique stones too. One of the best ways is to have a conversation with your stone dealer (eg me
) and try to figure out what you are sharpening, your style of sharpening, etc The stones up on Marks site are a sampling of stones I have or have tried, and I hope to increase this selection more an more, yet some will become available and then disappear as there are just so many of them around.