I'm starting to get less confident in the idea of a plane, for Beam NG at least, so is it better to start of a prop or vehicle as a cube? Or not?
Maybe you should ask this guy, he CLEARLY knows how to model cars imo https://www.beamng.com/threads/mgr-99s-3d-models-n-stuff.52503/
frankly, it doesn't really matter. The best way to do it for cars at least is to model a sort of "skin" of the car. Plane is probably easiest. Start with say the bonnet (that's where I like to start) and wrap your way around the car. Then once that's done you can cut sections out into body panels and then flesh them out. Personally I think a plane is the best way to start. What really matters though is the quality of the geometry and UV. How you go about making a mesh is solely up to you.