The only things that really matters on cheap bikes are the brakes and the forks. None of the other parts give any problems. The cheap shimano Tourney gears work really well and are extremely reliable. They're a good match for any electric bike with a hub-motor. The cheaper forks can give a harsh ride and the chrome tops will go rusty if you use it in winter and leave it outside. As long as it has disc brakes, they can be upgraded for as little as £40, so don't spend too much more for a bike that already has hydraulic ones unless you're scared of screwdrivers. Another problem you can get is badly built wheels, so always worth checking the spokes and adjust if necessary.
You can set the speed limit on that bike in the LCD settings and unplug the throttle if legality worries you. Like that, it's still not strictly legal, but I don't think anyone will care. You might be able to switch the throttle to dpendent on pedalling in the settings to make that legal.