Lets start with the supplemental braking system, in CA. we have a braking performance law. In other words, you have to be able to brake in a certain distance from a specified speed. It is virtually impossible for a CHP Officer to conduct that test. So in effect "NO" you are not required to have supplemental brakes.
Now, like most here, I feel it is best and safest.
If you are going to use magnetic brake/tail lights, DO NOT purchase them from HFT. Buy a really good set from your neighborhood RV dealer or from someplace like etrailer.com.
The magnets in the base of HFT lights are very weak and if you can't leave them on top(the safest place for them), those lights will fall off the sides of the bus. In fact if you want a free set of HFT lights, I'll give one to you. In addition to the weak magnets, the umbilical cable is very short and barely fit to the back of my Sidekick which is shorter than your bus.
On posting I see you asked about which system would be best. I suggest Ready Brake. No electronics, no vacuum motor. I'm not at all sure there is enough distance from the brake pedal arm to the front of the bus, if not go with Brake Buddy or any similar system.