This is just a really basic page I'm going to update with occasional thoughts about consumable selection. No particular order to it, just things as I discover them.
Selecting automotive filler wire
Assuming it's a standard carbon steel component, something in an ER70S-6 should prove sufficient for strength and will run on CO2 as a cheap gas. It has a higher silicon content than ER70S-3, so is good for welding anything with a dirtier surface (mill scale etc) so need to be aware of risk of silicon islands.
If you're dealing with one of the higher strength (high strengh low alloy, HSLA) steels, eg 600MPa or so, will need something to match the strength. Various versions of ER100S-G, ER110S-G, ER120S-G are available in a solid MAG wire, but might need a particular brand to be approved by car manufacturers. Metal cored or flux cored wires are also available, and something of the nature of an E110C-K4H4 might be suitable.