Definitely back-purge to prevent sugaring. And avoid the 75/25, go with the pure Argon.
As for rods, if you're looking to pick up something that can work on multiple projects, 309L will weld stainless-to-stainless as well as stainless-to-ms.
If you're just buying for this project, 308L is better for 304 to 304.
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