The Specialized website lists the rear hub cassette style with a "single speed conversion kit" so it sounds like you're in luck...unless it's a shorter spaced single speed casette hub. So I would double check whether you can fit a whole cassette onto your existing wheel.
BBGbashguard has bachguards for cheap and they work well. You will also probably need longer chainring bolts, unless your current ones were set up with spacers. Not a big deal at all, your LBS should have some for like 20 cents each. To keep the chain from falling off on the inside, n-gear jump stops work great and are also inexpensive.