Here is a little rpg mod for zandronum/lzdoom/gzdoom.... It has a levelup system where you get credits per level up and a buy sell system to buy items and upgrades, plus skills... It works like this...
As you level up you get credits and every 2 levelups you unlock a skill... The total amount of skills is 5...you need to bind keys to use the skills and the buy sell system... I bind the skills to f1-f5 and bind selet item to buy to q and buy selected item to e..
While in the game just hit q to cycle through items to buy.... Stat uprades are available also... Hit e to buy the selected item or stat upgrade...
Download link is here:
https://u.pcloud.link/publink/show?code=XZOcYkVZtosefRe8qOjS3KsAyM9yzVJ7NfBV
Should work with any other mod, provided you load it after said mod...
