This is going to sound obvious; measure the bus height. Use a ladder, a tape measure and a level, standing next to the bus; then when the level is equal to the height of the bus, measure from the level to the ground, and presto, the height.
Or the funny way is to try driving it through progressively lower doorways or under bridges until it hangs up; then make the garage door a bit taller.
This is a variation of finding out a bridge's capacity for the warning signs; keep running heavier trucks onto it until it collapses, then rebuild it and put the sign on.
DO NOT bother taking advice on how tall your bus SHOULD be. With the additions and changes you're going to make, it will change. So measure the bus, it's the only safe way.
The tool storage is nice, but where do I put the bed?