Just a thought without much to back it up...what about an EC155? If you get the B model you get the high HP 2C2 engines. It is plenty fast so the range might work out. Five main rotor blades should help at altitude and the bigger than the N3 cabin might be able to fit all your gear and people. Not sure about the purchase price or operating cost but they should be inline with the competion like the S76 etc. I'm sure EC can give you all the specifics but if you need any additional info let me know and I think I might be able to put ou in touch wit a useful resource!
Max