If you don't mind old graphics, I can suggest you the Fallout series. Best to start with is probably Fallout 2.
The theme is Post-Nuclear, so you'll find some mutations in this world.
It's cheap now, you can probably pick it up for 5 bucks.
Edit: You should try out 'Vampire: The Masquerade - Redemption' too. It's quite an action RPG but with a really nice story.
Came out 2000, so you get it for cheap now.