http://en.wikipedia.org/wiki/MV_Explorer_(2001)
That's about the only ship I can think of that would have the slightest chance of being a modern merchant raider. The MV Explorer, was a former Greek cruise ship of the now-defunct Royal Olympia Cruise Line. At one time, she and her sister ship were the fastest cruise ships in the world. She now serves as a school ship with the Semester at Sea program. I have no idea what happened to her sister ship. But at 28 knots, she could catch just about any merchant ship in the world, and have at least a slight chance of running from a surface warship. You could fit some weapons on her and probably even figure out how to put a helicopter on her stern. Again, I don't think the idea of a Q-ship has any place in modern naval warfare, but if I absolutely had to have one, that's what I'd use.