The location of the rust suggests that the car has had water coming into the passenger compartment, so when you get it fixed you might continue to watch for incoming water.
I'm starting out with new metal panels welded in from the top. On the driver's side it is an entire new aftermarket floor pan plus additional tie-in patches. On the passenger's side I'm just using patches as most of the pan is still good. A sheet of 18 gauge metal has been the most useful material for me so far, it's quite strong when welded into place. A tool that has been really useful for all of this patch fabrication work is an "air nibbler". This is an air compressor powered device that nibbles away at the metal as you push it along. This gives the overall effect of a power knife through metal, with little distortion of the metal as you might get with tin snips. Great tool.
Hobert also sells .240 welding wire for MIG work. Along with that you'd have to buy the right .240 adapter for the welding gun. I have not compared the .240 wire with the .030 wire on sheet metal welding, so I can only guess that the .240 wire is a little less likely to cause a burn-through hole. I did some welding with the .030 wire by joining two ends of short sections of square steel tube to make a longer piece. I'd say the .030 wire had good penetration on that work, perhaps that is the difference between the wire sizes.
Please be aware that the undercoating is flammable. Keep a fire extinguisher or water hose close by. I am in fact quite shocked that it is so flammable, both the undercoating and the materials used inside the passenger floor pan. Since I found that a propane torch is quite useful for softening and removing this undercoating, I have to balance the use of the torch with the knowledge the whole car could catch on fire if I'm not careful.
So be prepared with at least water.
Ken300D