Welder suggestions

Use the gas. Get a 75/25 argon/CO2 mix and as suggested run 023-.025 wire. Work with an infinite voltage machine if possible.

Get some scrap to practice on before tackling the side of you car. If you have an old door or some other panels laying around, drill holes in it the size that you are going to fill. Fill the holes using the welder and note the settings. Try to fill them in the horizontal position first. Then, stand the panel up to replicate the conditions when welding the car. The welder settings may require a slight tweak. Get comfortable welding in the vertical position, then go after the real car panels.

Try starting at the top of the hole and fill down, bottom and work/layer up, side, etc. Find the method that works best for you. I like bottom to top.

If you can get a piece of copper behind the holes to back it, it will help in reducing the chance for burn through. The cleaner you can make the initial weld, reduces body work required to get it straight.