Our SPIFR EMS EC135 has an empty weight of 1882kgs.(max gross weight 2835kgs) It would have a useful load of 953kgs.
If we subtract 134kgs EMS equipment from the useful load, it would leave about 1087kgs useable.
If you wish to fly with 2 pilots and 4 passengers, figured at 200 lbs each, that's roughly 540kgs. So subtracting 540 from 1087 leaves about 547kgs left.
Since you are flying offshore, floats would have to be added which I am estimating at around 100kgs. That leaves about 447kgs for fuel. The fuel system holds 448kgs in the mains and an additional 92kgs in the supply tanks for a total fuel weight of 540kgs with full fuel.
So as you can see, that extra pilot is holding you back. With a full IFR package coupled to an autopilot system is the extra pilot neccessary?
Since a dual pilot IFR ship would probably weight more than the example I am using, and interior requirements such as seats, passenger baggage,etc would also have to be added, I would recommend you look long and hard at whether or not it would meet your requirements.
An EC135 is a great ship, but it has it's limits.