You'll have to do a lot more cutting before you can start thinking about welding. With holes that size you typically find that MUCH more of the internal structure will be affected... Mine was better than that and is currently missing its entire sill and inner+outer wheel arches.
1.5 is usually reserved for the thickest structural metal. 0.8mm is more typical for random panels. I use 1mm zincor sheet for most things as it is more forgiving.
A weld = heat. Even a spot weld needs to melt the underlying metal to be effective and take any paint with it.
However, there seems to be severe paint system failure in that area..I would be very suspicious of that seam having more severe rust on the inside. You need to chop off the bottom of that panel anyway, take a look up inside all the seams with a flash-light and see what you're dealing with. Any rusting along those seams will be next years rust hole. Because the panels are so thin to start with, any surface rust means either complete replacement or being left with 0.5mm thick panel sections after cleaning up (won't last long).
Good luck with MIG practice, it can be a steep learning curve on thin sheet-metal.
Thanks for the advice. There is, what looks like, superficial rust going all the way aroun the arch on the inside, however it looks superficial. Not sure if I should cut it, I can possibly cure it with waxoyl?
My welder is not here just yet. I intend to practice on some old metal door from a wood burner, got some cracks in it. I will post updates when it's here so you guys can roast me