The problem with flux core for bodywork is that you want to put a patch on completely with tacks - no continuous beads. So the flux core will leave a flux coating on each bead which would have to be thoroughly cleaned before you could put one next to it. It would take forever to put on even a small patch !!

.023 is ok for for tacks but for plug welding I prefer .030 (personal preference)

You definitely want to use the argon/co2 mix.

Look into the wire - apparently there is a wire that yields a softer bead, making grinding much easier and therefore generating less heat. I haven't found it yet but haven't looked that hard either!