Making a bus 4x4 will take around $10,000 alone to do properly.
If you can find a 4x4 three ton truck, it can be done cheaper, but they are rare.
After you convert it to 4x4, top speed will be 70 to 80 kmh max.
Things that big need to be 6x6 or 8x8 due to the weight and height for soft ground. The tandem rear gives more traction, and keeps one tire from falling into a hole, causing the bus to tip over. In a 8x8 configuration, the second tandem front makes the front act the same as the rear.
Pirate 4x4 is where I recommend you look for info. More large 4x4 builds there.
However don't go over there asking questions. They will rip you apart. Search, read and search more.
I would use 2.5 ton top loading rockwells as axles, or portal tech.
A NP205 divorced transfer case would be strong enough. It's a gear driven, cast iron case.
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