Since North Korea claims to have developed a sub-launched missile, the
unit most probably will be a a 4 to 6 tube SSBN, a capability consistent
with what they need to project their threat potential. An SSN would not
serve that purpose.
Patrol area: most probably between NK and there Chinese mainland,
at least initially. I don't see them patrolling in the open Pacific to threaten
the US.
It may be time for a review of all that is known about acoustic detections
of submarine disasters.
Would not be surprised if they make use of first-generation Chinese SSBN
design and technology.