Forks also come with different diameter stanchions (the upper portion of the fork
that slides inside the lower portion) ranging from about 32mm up to over 40mm. Generally the bigger the stanchions the stiffer the fork, which will help prevent the fork from flexing when using the front brake. Because the bike will be quite heavy after all the components are installed it's best to get a very stiff fork that can handle a large 8-inch brake rotor. There are several different types of axles for mounting the wheel to the fork. You want a 20mm through axle, which usually comes on the larger, heavier duty forks. The larger axle will help to further increase stiffness of the fork and is definitely recommended for a high-powered ebike. Some forks also have dual crowns, one below the steer tube and one above it. This also makes the fork stiffer but I don't believe it is necessary as the newer single crown forks are more than adequate for ebike use.