The Marines already have a tailor made solution to the issue of what to buy for the Marine Personnel Carrier. That solution is in production and being delivered today. It would need minor modifications and is transportable by CH-53E/K.
Its the LAV-25A2.
Simply remove the turret, replace it with a remote weapon station and the issue is resolved. Approval by the SecDef should be easy, it can be sole sourced (by passing competition) and it can be in the fleet by the 1st quarter of next year. IF they act now.
USMC LAV A2