the tallman is right about the strings and action.
A lot of times, cheap guitars sound great for playing slide (Hounddog Tayler). Another thing to think about is the shape of the fretboard. Some are very flat while others are more round. For slide, you'll want a flat fretboard.
About differant types of slides, if you're going to play electric, you'll really need a glass one. The metal ones sound good until you start to play loud and then you start to get a raspy sound. With glass, it's not a problem. I like the heavy ones with a thick wall.
On the other hand, if your playing accoustic, I'd suggest a heavy
brass slide. You can't whip it around as fast, but it's a lot less work to get a clear sound.
One more thing, if your just starting out, put the slide on your little finger and let your other fingers lay on the strings behind the slide. This will reduce a lot of excess noise and give you a good clear sound. Keep in mind, this is one finger fretless guitar.