If you are going to get into naturals I think your best bet is to find a trusted someone who can vouch for the qualities you are looking for before purchase. You will be rock collecting otherwise.
If you chose the path of synthetics the naniwa 12k may be your next step without much wallet damage. If wallet be damned I suggest Suehiro Gokumyo #20000 grit (@ 0.5 µm). : Tools from Japan, Japanese woodworking tools direct from Japan. you will be pleased.