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For razor honing I don't know if it matters that much, touhh I admit the 4K side wears down enough that the surface needs to be refreshed after each razor imo. Still, the 4k/8k is a good stone, and...

If you've already bought the 16K, then it'd depend on the money you are willing to spend.
The first ting I'd buy is a DMT8C for lapping it.

Then I'd buy the 4000 and 8000 shapton
OR
a norton...

DMT makes diamond coated plates.
The DMT8C is 325 grit rated, and ideal for flattening and maintaining all kinds of waterstones.
The DMT8E is 1200 grit rated and used by some to set bevels.
The...

1) that is up to you. The norton 4k/8k is also a good stone, and some people use the DMT8EE instead.
2) yes. But I don't know if that is useful or not.
3) No. A DMT8C will be just as good for what...