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!
I agree with most of your statement, however my personal preference is .035 wire, and just Argon gas, as far as grinding the welds, I find a 36 grit Zirconia flap wheel the best, but change often for a true flat cut
