Welder suggestions

You left it wide open as to what type of machine you have access to, not all electric welders are appropriate for welding sheet metal.

The most common welder for working on sheet metal is a MIG unit. For filling holes 0.023" wire is going to be the ideal size. The 120 volt units have plenty of capacity for most anything on a car, just use the machines recomendation for settings and practice on some scrap metal, you can fine tune the settings as you get used to working with the machine.

TIG is another process that works well on sheet metal but requires much more skill (practice) to become proficient. The gasless MIG welders are not a good choice since the flux core wire used with this type of machine tends to blow holes in thin sheet metal. And of course a stick welder is totally inappropriate.