I'd suggesting thinking long and hard about your mission. How are you going to use your bus/home? We are all different with different goals and different ways of living - there is no single "right" answer - only what works best for you.
If you have no experience with the lifestyle, rent a small motorhome for a couple weekends and get a little experience. It is impossible to know what you will find important without some experience. For example; the size of the holding tanks will mean nothing to you until you have some idea how much water you use and how often you have to make a run to the dump station. Or, if always hooked up to water/sewer - you may not even need holding tanks. All part of the "mission".