"Bargain Buys" Most electric bikes priced less than $600 at big box retailers and on-line are aimed at the kid/teen/toy market. They generally lack the performance and durability that people want and expect. Also, parts and service can be problematic with both big box retailers and on-line vendors. We urge you to invest in a quality e-bike, preferably from a local dealer, that will serve you (and others) for many years. Remember, if it's poorly constructed and you can't get repair parts, it will likely become land-fill material. Save, beg, or borrow the money to get a quality bike or kit.
E-bikes mostly use motors and battery options from a few major suppliers: Bosch, Yamaha, Shimano, and Brose. A few other brands exist, but are less reliable or powerful. Some, like the Yamaha system, have more torque and others are quieter. But generally all four make good options. Look for motor output (in watts) which will give you an idea of total power. But watt hours (Wh) is perhaps a better figure to use—it takes into account battery output and life to give a truer reflection of power.