Oven cleaner works OK on floor panels. Don't sniff it
A mild solution of pool acid and water can do nice work on that rust as long as you keep it mild
Some paint can be left on if you want
Don't go crazy with the wire brush wheel, wear goggles and a face shield
The rust spots will win against just a welder, you need to make patches even if they are just little
patches. Rust burns, it does not weld.
Wire wheels have they're place, in corners and on rust.
It would take forever to do the whole body and make a mess
A 15 dollar can of paint remover at Home Depot will go along way towards stripping off that old paint