The Mursame Design was further developed into the Takanami class, which in turn has now developed into the Akizuki class now in production.
The Takanami has much of the development you mention, adding the OTO Melara 127mm gun and Mk 41 vls, so maybe that would be a better starting point.
But then, maybe the Akizuki would be a better starting point - but info on that class is still kinda thin.
All have a crew compliment of 200 or less and are about the same size.
Whichever platform forms the base, I would make a double helo hangar a priority. Not just for a second helo, but for the flexibility that space provides - the second hangar could be considered a 'mission bay' (in current parlance), it could handle a second Helo, or could handle multiple UAVs. Easy to modularize extra command facilities, SPECWAR berthing/mission equipment, collection equipment, or any of a number of other items.
Another option for baseline:
http://www.defenseindustrydaily.com/gib ... led-02510/