1) There are a total of about 5 missions on it. There is Hagar Caves, Refinery, Scorcher Base, and a few other levels. They are also great and even add a few new gameplay varietys in some sections such as opening doors with valves hidden in a control room. It takes about 3 hours to beat.
2) No new abilitys, but there is a new mode called Scenario, a new weapon called the Nail-Gun with several different types of ammo, and Ultra-Nightmare mode. Also, the ability to keep playing after you beat the game.
3)I would highly reccomend it. It's only $5 so support the devs if you can.