Only Dead Space 2 and 3 have DLC on the PC. For whatever reason, EA left the original Dead Space "dead in the water" in regards to dlc.
An official patch came out a few years back that gave PC gamers access to the 3 DLC packs of Dead Space 2 for free. It seriously cheesed off the console gamers who had to buy them for $5 apiece. For that matter, they still cost $5 each on the consoles. EA could at least make them free now.