Ok, so let me first write down what I want to do with the bus so that you can get an idea for my reasoning. Since I live in Alberta, I am very close to the Canadian Rockies and would love to spend time exploring them further than I have been able to on day trips, so I am looking for an engine that won't conk out while driving through the high mountain passes. If its strong enough to tow a little run-around vehicle then that's great too because it means I won't have to drive the bus for every little errand.
Another thing is being able to start the engine in the cold. Diesel is obviously a lot harder to start in the frigid temperatures of the far north that I currently call home than petrol but diesel is a lot more efficient for very large vehicles. After some research, I have found that some diesel engines are easier to start than others in the cold.
From my current research, I have found that the best option seems to be the International DT466 engine. It seems to be the most powerful without compromising efficiency.
Another thing I have learnt to consider is the transmission you have in the bus. An automatic transmission seems like the best way to go for me and would be easier for others to drive if that is ever required. As for the possible automatic transmissions, it seems the Allison MT643 transmission is the most recommended because it has the ability to lock up at high gear. While originally this sounded like a bad thing to me, it allows the transmission to transfer input energy to the output shafts while minimising the wear on the transmission because there is little to no slippage.
I'm going to keep looking into this but this is my initial research. If you have any other suggestions, please let me know, I would be grateful for any suggestions!
Hope you have a great time getting out on adventures.