I use TIG as much as I can. Way easier to clean up after than MIG.
Rule of thumb: Match tungsten, rod, and metal as a starting point.
If you have DC lift arc tig, it doesn't like the .040 tungsten. It sticks while trying to start the arc. But with HF start, .040 makes a nice tight HAZ! So with lift arc, I would start with a sharp 1/16" tungsten.
No filler will leave a small undercut. .023" wire vaporizes too quickly before dipping into the puddle. .035" filler is just about right. .045" wire is too big and will cool the puddle, and leave a lot more to grind off.
Pedal will be way easier than torch switch. You need to back off the heat as you go with sheet metal.
Great video on TIG welding patches:
https://www.youtube.com/watch?v=NZ_S-7ZSvHk
Cheap tungsten grinder:
https://www.harborfreight.com/multip...ner-99823.html