Are you planning to keep the rest of the electrical system as it was? Whether you are or not, make a list of the 12V
equipment you expect to run, figure out how much current each will use, and add it all up. If you're planning to replace the light fixtures with LEDs then that will likely reduce the overall load requirement, but adding a bunch of 12V
to USB charging ports would push the load higher.
If you're looking to replace the fuse panel, then you should figure out a logical grouping of loads (maybe more than the four that my Univolt had) so that you are unlikely to blow fuses simply from momentary higher loads such as a fan starting.