Needing or not an Instrument Helicopter rating is obviously up to you, you most definitely need one though to fly under IFR in that category and class as an Instrument Airplane rating won't be legal. It takes you 15 hours of dual Instrument instruction in a helicopter for your add-on rating, provided you already meet the other aeronautical experience requirements such as 50 hours x-country PIC. The reality of helicopter flying outside of the US is a bit different, but there's no such thing as a Helicopter Multi-Engine Instrument rating in the FAA system and you can still fly overseas using your FAA certificate so I wouldn't worry too much about the other guys are saying. Hope this helps!