From the pics the floor is not unfixable, in fact none of it is unfixable. If the weird pillar/window rusted area is localized to just one side and the rest of the car is relatively structurally sound I'd repair it. 6" worth of repair panel for the floor or 2' it doesn't really matter once repaired correctly. Your not going to see it so it's much easier than exterior stuff as far as your skills go. I wouldn't let that scare you away. Lots of these around with good pillar areas to patch if it's not around the complete car. Inspect carefully and when viewing visible rusted areas times what you can see x2 and go from there. Last time I seen rust much like that was when a car was parked next to a building with roof eaves runoff cascading on it for several years in a heavily wooded area. The sun never hit it and debris, sap, etc. collected there and held in the moisture.