Some confusion here.
2 distinct needs:
1. ASW with all the sub-hunting and killing systems associated with the role.
2. Strategic reconnaissance. These days that can be 'smart sensor, dumb platform', so if flexibility is needed, any airliner-type platform with adequate electrical power supplies and endurance would suffice. Which tends to favour an ETOPS twin, largely gutted then fitted with the relevant sensors and as many pie-eaters as necessary to operate them.
A300B4, perhaps?