Trading is extremely important. I also feel as though it would be best to learn the ADC position with a champion with no mobility. You will get much better at positioning this way, and once you learn the role and you choose which ADC you want to main you won't have to rely on their mobility to survive because you will be so used to living without mobility. This will make you a better player than those that overly rely on their mobility. You will notice that a lot of Ezreal mains aren't as good at positioning because they are used to playing a champion that can just flash away. You will also notice Ashe mains are damn good at positioning because they if they don't position correctly they die.
So, because Ashe is being reworked and she will be the only ADC that doesn't rely on RNG to crit, I would suggest learning with Varus. No mobility. Great poke. Great champion to learn on imo. You will learn quickly how important trading and positioning is as him.