Since I may want to use the characters in a commercial setting, I do want to know if free means either that I don’t have to pay or that I can do anything I want with the pic (with some restrictions that may or not apply), so the terms of service matter to me 🙂 ). Gratis vs Libre 🙂

A RPG is as simple or as complex as you want it to be.

Since your world is basically ready, it’s all about the rules, not about the setting. And the rules can be one line long:

“Throw a coin, head wins, tail looses, GM decides if he needs to, period 😉 ”

Ok, I’m pushing the point for the sake of argument but this is actually true, I actually played and GMed without any rules. Roleplaying is about roleplaying, not about the rules. 🙂

Now you could use that tool it’s an interesting system (not a d20 copy by far btw, it’s from 1983 ;P but it’s in french so use your friendly google auto-translator 😛 😉 ) : https://fr.wikipedia.org/wiki/Charte_angoumoise