Firstly, they have to build a settlement! 'WHAT?' you may ask, but it's true. I've written rules for the players to build and control their own settlement.
Second, they have a gang leader. I've written rules for challenging the gang leader.
Thirdly, availability of weapons and their prices. Yes, you've guessed it, I have rules for this as well.
I will update on this amazingly (well stupid) idea for a GM and again, if you want a copy of these rules for play testing, please, ask.