Wire thickness is based on amps, distance, and how much power you're willing to lose (as heat, of course) along the way. You can save half your money by having your power center in the middle of the bus so none of your runs are actually 40 ft.
I use DC for basically everything, but I do have an inverter for infrequently used things. it has fuses on the input side so it's connected directly with a nice thick cable.
For the rest of my build, I have a fuse panel: https://amzn.to/2A7ZRWs
I tie my start and house batteries to a shared chassis ground, so I'm only running positive wires.
I don't see a good reason to have a breaker combined with a fuse, but if I did it anyway, I'd use a higher rated fuse and a lower rated breaker. Note also that most household type breakers are NOT suitable for DC power.