For my first car i bought a 1990 XJ 4x4 for $800 vehicle is a little banged up but was def worth the money,

while checking out the car i noticed major amounts of rust under the vehicle as it was his beach car but i thought it was just the under layer. I was wrong there is only one layer and in some places there is only carpet, i did some research and i don't know if it would be earlier to weld or replace it is only rusted in the trunk and rear p***enger side area i want to fix because im scared something is going to fall through

any help would be sweet thanks