That's the problem though. The players will influence the Alchemist's decisions. He's looking for people because he thinks if he taught someone his skills then they could help him run his shop. He's a very lonely man who has to talk to puppets all day since they're the only people who like him. Think of the poor man. If he did have a person enter his shop, he'd show them to the different pharmaceutical drugs and let them pick. These people actually know that he is a true alchemist and learn his secrets and information about the man. That is the hook of the whole roleplay. You're constantly learning about this man and alchemy and you can make decisions to effect the whole scenario. Will you tell everyone the secrets of alchemy? Will you leave the man out of your life? Will you make potions for your own benefit or will you be kind and help the unfortunate guy out?

FYI "The roleplay is set in a big town (possibly in England or maybe even a parallel planet to Earth?) and the time is present day" I'm imagining London or Bristol but prettier :3 The alchemist's shop is going to be small and near a boring corner shop. Maybe a cafe or a bank, ideas are welcomed.