You would never buy a car online and assemble it yourself. E-bikes are also complex machines that should be built and checked by professionals. Your local bike shop is the best place to get started safely with your new e-bike.
Riding an e-bike bike places loads of stress on the frame, fork, brakes, and wheels. You need higher quality components to ensure a smooth stable ride when riding at speed, and to help you be able to stop on a dime, in rain or shine.