So to replace the Leanders, we are basically looking at OPVs, right?
So what about something like the Patrol Frigate 4921 from Ingalls be feasible or bit too large and expensive? And a Gowind would be relatively feasible right?
Which is more frigate like version of the National Security Cutter you mentioned IIRC
Yep, my guess is you are looking at large OPVs or small frigates. The appeal of an OPV is range and a general focus on reduction of manning costs, while a frigate will offer greater capabilities and speed, with reduced range.
Huntington Ingalls offers two version of the NSC hull. The first is the Patrol Frigate 4921 which adds improved radar, an option for an enclosed mast, 18 SAMs, Sonar, torpeados, and some SSMs. The OPV 4501 loses these in exchange for keeping the bow thruster, the full 12000 nm range, and the stern RHIB ramp.