You can tee off the regulator line and just use a hose with a ball valve to regulate the purge flow. You will have to listen to the flow and adjust the regulator and ball valve. Its not exact, but does save from an dual regulator if it is not something you do regularly. I used this method for Tig welding some stainless exhaust and turbo pipes.

Solar flux works great, but I wouldnt use it inside a gas tank.

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.