My 2 cents on this thread,
personally I wouldn’t upgrade to a larger coach/engine combination unless it has a engine brake, especially if you are towing heavy as stated, if you ask the current owner if it has an engine brake or an exhaust brake (hopefully he knows the difference) it might determine what power plant it really has, I don’t believe ISC’s had the engine brake as an option, only exhaust brakes, to my understanding, If it’s a transplanted engine it might not have an engine brake( lots of reprogramming with transmission and what not), I would think that if it has the engine brake it most likely was built that way. Jmo.