When selecting a suspension fork for your electric bike you need to make sure it is the recommended length to match the frame it is going on. If you purchase a complete bike the fork that comes with it will already be a good match. A fork that is too tall for the frame will change the handling characteristics of the bike and add more stress to the head tube than it was designed for. This could result in the frame eventually cracking from the added leverage. It will also raise the stand over height of the bike, which could make the bike to high for you to comfortable sit on. A fork that is to short will lower the front end, which will effectively increase the head angle and change the way the bike handles.
Some forks have adjustable travel, which can help you dial in the correct height for the frame, but they tend to be more expensive and not really necessary on an electric bike. Just get a fork the same height the frame was designed for. Check the frame manufacturer's website to see what size fork they recommend on the particular frame you have. Forks can have an air spring or a coil spring and either will work fine. An air-sprung fork can be adjusted using a shock pump to change the air pressure inside the fork. A coil-sprung fork may require changing out the internal coil spring to a heavier one to match the increased weight of the finished bike.
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.