Is it any way to know the ammount of gold or other resource a player has in his inventory?
Not directly. You can either create a variable and make it higher each time gold is earned (for example, when a gold coin worth 200 gold is collected you set the player gold to 200). This doesn’t work for multiple round/game saving. What does work is using score and the GetCurrentScore function of the score manager
Yes. I know exactly what you suggest, but as you say it can be a mess with persistent data. 
Player marker combined with a conditional button could solve this.