If you have a good diamond stone, sharpening should be fine. It might take a little longer than other premium steels, but not too bad. It is much better to not let it get dull, though, but that's a suggestion for any premium steel done at a higher RC (I'd say 60+).
I'm not sure why I don't get much discoloration on my Delica/Endura ZDP blades, but my Lum Chinese ZDPs sure seemed to develop a stain quickly. They didn't develop rust or pitting, just seemed to stain a little. Other than that, I think ZDP-189 is probably the best SS (For edge holding) for a folder that's available in a production knife.
There might be some better steels out there, but not one that's readily available for production knives.