These are typical rust areas. That fender doesn't really look too bad. Both of mine had about 10 lbs of filler in that area and I am not exagerating.
Hard to say just from looking at the picture but, on the fender I'd probably grind to bare metal, drill out the holes and use a sheet metal punch to make a disc that will fit in drilled hole. Weld it, smooth it. This is a good technique for metal that's not too thin yet from corrosion and has random holes. I'd guess a lot of that area on fender is right over where the inside brace is, where dirt and moisture gets trapped.
You can get cab corners and lower door skins from the parts places.
That floor area next to the seat looks like it probably has some pretty thin metal. You'll probably have to cut that out and weld new metal.
On my truck, I did a combination of replacing metal and using filler with hair. It's not a show truck and I wanted to drive it in a relatively short period of time. If the filler areas last 2 years (a reasonable expectation when it's not a daily driver and is garaged) I'll be happy. I'm having fun driving it to car events. More fun than if it was still all torn apart having all the metal replaced.
The areas where I did the filler, I cleaned up good and then applied a paint called Rust Bullet which thus far, I've had good luck with. It's tough stuff and provides a good barrier. Even on the areas where I replaced metal, I took extra precaution to try and avoid repeat rust out. Some spots I used Rust Bullet or an anti-corrosion compound from Eastwood or both. The Eastwood stuff comes in a spray can and is a oily, waxy substance that is good for inside doors and rockers.
You've just gotta decide what you're going for with the truck. Restoring, show truck, just making it a good driver? And then get to work.
-David
