C-25 SHIELDING GAS (75% Argon, 25% CO2 ) as mentioned is the best if your working w/mild steel. Which is what mostly all cars are (mild steel gauge < 18).
I'm glad to see your at least going to use Gas. Many people start out wanting to use Flux Core wire (gasless). While it's do-able the heat needed is higher so panel warping and blow-throughs are common. Not to mention there is a heck of a lot more grinding to do to finish up.
Your probably better off renting the bottle from a local welding supplier as many won't fill bottles/cylinders you bring in from somewhere else. Even if it's new. Sometimes that's based on size.
Something to do with their liability in case of explosion from a faulty one and each is supposed to be re-certified after so many years. I think 5 or something like that.
Greg