Repair or Replace?

Now I can see some of the problems that were hidden under the slime

The 1st one is an easy decision I'm going to need to replace the plywood at the bottom of the Gas bottle store on the back as it's rotten. This should be a straightforward job of cutting a new piece of ply and fitting it. I will "upgrade" from the current piece by actually painting it so it doesn't get wet and rot away again

The second is a little harder to decide. The door pillars on the front are obviously rusted as you can see in the picture below

I need to decide whether to patch with filler, get panel welded in or have a section cut-out and replaced. It's not straightforward in my mind as I have no welder so I would need to pay for someone to do the job.

I will find out whether this is structural and whether a replacement section is easy to fit so I will ask around before making a final decision