I may not know much about Defenders but I think the seat base is made of aluminum. Replacements for that part are easy to come by as well as the floors. The foot wells are another thing all together and may require complete disassembly of the majority of the vehicle to expose the bulkhead to properly repair. However, new bulkheads are also available. In fact there aren't too many major body components that you can't get replacements for.
Thing is with Rovers that a small area of corrosion that is visible means that there are lots and lots of other areas that are not visible that are equally if not more corroded. If you don't want to get involved with a complete money pit then likely best to move on.
1966 109 5 door wagon 300Tdi "spermaceti fueled"
1994 RRC LeWiB "ruining the air behind me"
All my troubles are Rover