I owned and operated three PA-31-350's in the 80's along with 9 other aircraft on a Part 135. I flew the aircraft all over South America, Alaska, Central American and Alaska. If I had to choose a piston aircraft to go anywhere and do anything I would choose the PA-31-350. It is great on rough strips, I flew mine out of Naknek and Pilot Point in Alaska. I have flown it out of Bogota, over the Andes and into Jungle Airstrips. It will haul an amazing load and will fly way over gross with no problem at all, loose and engine and you are looking for a place to land but it has amazing performance. With an engine out at legal gross 7000 lbs. it will climb at 200 fpm at 80 degrees F. It will do a lot better than that at cold temps. With the turbos it will give you great single engine climb even at the higher elevations.
Fuel burn 35 to 40 GPH depending on how you run it.
Maintenance parts only about 20.00 per hour
Engine reserve comes to 40.00 per hour
200 man hours to perform an annual or 100 hour inspection. That would come to about 50 hours of shop time.
I would think that the lease cost now days would make it an impossible airplane to run Part 135, Insurance is crazy too.