I've not tried any of the HF welders but assuming they're capable of doing the job, I question your choice of a flux core welder.The sheet metal on Z's is very thin. A flux core weld isn't as precise as hard wire. There is a lot more smoke to prevent good vision, (even on a spot weld), the weld is left with slag on it that doesn't conduct elec. very well and you'll be left with a lot of splatter to clean up. Hard wire, on the other hand will allow smaller, more precise welds on thin steel. CO2-Argonne mix is expensive but is the easiest to weld with and will leave you with the least amount of clean up (no slag) and little splatter. I use straight CO2. Quite cheap, not quite as good as the mix and leaves just a little splatter to clean up. I've got quite a bit of experience with both types and while flux core is good for some things, rusty THIN sheet metal in tight places isn't one of them. Also invest in an auto-darkening hood. You'll need it to start small welds in the right place.