So much of this depends on the extent to which something is rusted. Rusty stuff is fairly easy to remove compared to solid metal.
I would spot repair only if it was rusty in a certain smaller area. If the whole floor pan is rusty then it should be replaced as one piece. If a small area is rusty, then I would make a small patch and repair it that way. If the front section of a floor pan was rusty but the back was good, I might cut out all the rusty stuff and then cut the new pan in two using only the pieces that I need. It's kind of a 50% rule that I look at and as long as it will be a good repair when done, I'm OK with it.
In the case of the area that ties into the side member at the back end ( in front of the rear wheel ) I would make all of that by hand. You can buy a patch panel for that area but again, I would only use what's needed rather than cut everything out.
In the case of the side members or sills, if they're only a bit rusty front and rear but not really into the jack ports, I would spot repair those areas. If the whole bottom of the sill is rusted away front to back, then it's a matter of a lot of work. Don't let anyone fool you into thinking it's an easy job - it's not....
