Thanks to clever engineering, the Electric Brompton folds up identically to the non-powered variety. It is heavier than a standard Brompton of course, but quite light by e-bike standards. It has small , suitcase-style additional wheels that come into play once folded up, and the way the weight is balanced means it is quite straightforward to trundle along in that state.  
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.