98/2 is a common shielding gas for carbon steel, as is 75/25. As for wire quality, your wire would depend on what type of metal you're welding. The AWS Specification for filler metal for carbon steel is A5.18, but this specification will change with the different types of metal you're welding. Shielding gases for different types of metals can vary from 100% argon, to an argon/helium mixture, to a helium/argon/carbon dioxide mixture.