My thoughts are that if you buy a working bus conversion it will already have water, sewer, electrical and other items already done. You don't sound like you want to install sewer tanks under a bus. What I'm saying is if the heavy work, like insulation, plumbing and heat are already done you could easily change the interior to suite your needs. Honestly the floorplans of most buses are remarkably similar because we can't get away from that bowling alley down the middle. There just aren't that many different ways a person can arrange the interior. Your floor plan will likely be similar to others here because there are only so many places you can hang waste tanks on a bus that pre-decide where your bathroom will be.
You'll run into floorplan issues, out of necessity, even if you have someone else build your bus.
Good luck with whatever you decide.