I just replaced my drivers floor panel, but it wasn't in nearly that bad a shape. Well, I mean, it was, but it was just the panel itself, and it didn't extend to my roll bar mount and my torque box was just fine.
New panels aren't all that bad -
here's a set of 4 for $230 - they are high quality and fit like a glove (want to guess what I used?). They sell them individually as well, along with the torque boxes if you need them. The issue comes when your rust extends beyond the panel itself. With enough time you can fix anything, as noted, but it will get expensive quickly.
I don't weld. I used 3M panel adhesive and stainless, closed pop rivets to hold it all in place. Just adding that panel made my TJ much more solid than it was. You don't need to go that way since you MiG, but for those who don't, it is a full day job, but any idiot (like me) can do it with a grinder, pop rivets, and adhesive.
Doug