If you can get external or internal overload tanks for a 117, it's an option. MAUW is 3350kg, expect about 2000 - 2100kg empty for a float equipped machine. Fuel burn is 220kg/hr at altitude, cruise speed about 125kias, but may drag back a bit with floats and external tanks.
So, disposable load will be about 1300kg, less 4 p.o.b. 400kg, leaves 900kg for fuel, 4hrs. Standard internal fuel is only 568kg useable, you'd need to find an extra 250kg or so of overload tank to make your 3.5hr requirement. Not difficult, there are a number of 117's around now with external tanks, and with only a few pax inside, there would be heaps of cabin or boot space to install an internal tank.
Or even an S76, for a few more dollar$
