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.

in my area in centeral ar. the 2 most common gasses to weld mild steel
with is 75argon 25 co2 and 90 ar. 10 co2.
the lower co2 gases do a better job and can go into the spray mode
where as the 25% co2 will not spray with .035 wire
most mild steels are welded with the mig process are e70s-3 and e70s-6
i prefer the less co2 mix gas my self.