Icyghosts

My Friend and I want to run a "Learn to Play" Age of Sigmar event at our local gaming convention Wagoncon. I'm looking for advice from anyone who has run a Learn to Play previously. Our current plan is to get 4-6 start collecting boxes, paint them up, let people pick an army to play with, teach them the rules and mechanics of the game, then raffle off the painted armies to the people who played. Is this a reasonable way to go about it? Start collecting boxes may be too many points to try and learn the game with and may make games go too long. What is a good list sizes/army to start with? How much time is reasonable to allocate to each match in hopes of that each player walks away with a better understanding of the game and has fun?