Turning a merchant ship into a peer warship means rebuilding it

What work and rough cost increase would turn a civilian ship into a combatant able to challenge a contemporary warship of similar size?

So much combat-system and structural reconstruction that the project approaches building a purpose-designed warship.

A civilian ship capable of challenging a contemporary warship would need extensive internal reconstruction to install combat computers and sensors. Weapon mounting points would also require structural changes to the hull. The size comparison needs care because most seagoing merchant vessels are larger than most warships. An initial comparison with Star Wars MC80 conversions leads to a clarification that the question concerns present-day ships on Earth. The response favors building a warship for that requirement and gives no numerical cost-escalation percentage.