Chevy Blazer 1988 K5 floor completed rusted and seems weak and brittle
 

I recently purchased chevy blazer 1988 and noted the floors were heavily rusted and seemed especially rusted and weakened on front driver section as it was tearing out down as I placed heavy pressure and even the seats floor which were bolted were rusted and tearing out. I also saw rust on the floor sides corner jointed at door area

I was thinking to change certain sections of floorings wherever it is rusted and need advice whether I should change various section or change the whole floor as I will be doing heavy offroading with it .

Any idea what material I should use for changing floor or floor section.

Would appreciate advice if i should change sections or change the whole floor