Most any welder will work if you know how to use it, but arc is fine.
Carguy is correct, most large dents will need to be pulled more than once sometimes several different places. Went dents form metal around the dents strech they don't just cave. If you plan to try to pull the dent you must pull in several different spots as well as shrink the steel back into shape. There are a few methods of doing this but none are for the beginner. If you try to pull the dent and do it in several different places try dry ice. Pass a chunk of dry ice by the outside of the dented area, the dry ice will cause the steel to contract and sometime the dent will pop itself out depending on size, direction of dent and place on the body. This is the only "safe" way to do this without the aid of an exsperienced bodyman. Other methods are normally used, but to the newcomer they are not recomended do to difficulty.