When I turn the key, the engine makes a horrible scraping noise

Sounds like a bad solenoid. Your starter consists of two parts, the starter motor itself and the solenoid. The starter motor is just what it sounds like, a motor big enough to spin the engine and start the whole ignition process. It works by turning a small gear quickly against a large gear on the flywheel of the engine. But that small gear isn't always engaged with the flywheel. That's where the solenoid comes in. It basically sticks out the smaller gear to engage it with the flywheel. When it starts to go out, it doesn't engage properly and makes the noise you've been hearing. And in order to protect the flywheel gear, which is much more expensive to replace, the solenoid gear is usually made of a softer metal, so it wears down. The solution is to replace the starter.