Quote: "1. A leveling system for characters? Can you level up the characters after they get to a certain amount of experience points?"
yes. this can be done with HUDS and certian scripts to give the player extra privelages when leveled. I would use project blue mod.
Quote: "2. Can a money or gold accumulation system be implemented?"
yes. fenix mod can do it quite easily.
Quote: "3. Is there a way to implement a backpack or inventory system?"
yes. using huds and scripting, preferably of the project blue variety.
Quote: "4. Is there a way to sell these items that you collect?"
yes. once again, fenix mod or project blue mod
Quote: "5. is there a way to level up enemies as you level up?"
well, you can make enemies more and more powerful the more you level up, if that's what you're asking.
Quote: "6. Is there a way to create a party?"
like mmorpg WoW parties? or clans in online games? nope, sorry.
Quote: "7. Is there a way to do a create character system?"
technically yes, but it would be one of the most scripting-rich and media-rich projects that could be done.
all in all, buy project blue mod for the most scripting freedom to do all of this, or you can get fenix mod for free, but with less commands it's not going to be quite as extensive as project blue.