Cab Rust Question

[andyh1956]

Boys I have a question on how to repair a rust spot. I have been shopping for a pickup for my grand son & looked at a better than average '83 short wide C10 today. It's brown w/ a cream colored roof & Beige interior. 250 p/s p/b column shift. One repaint after a driver door accident.
Under the cab is perfect. inner / outer rockers, floor, floor supports & under corners are perfect. Door bottoms perfect. The owner is 88 yrs old & has let the truck set out under trees since he's owned it & on the driver's side inside of the door well, let's say in the crease where the seam sealer goes from the striker up to the curve that starts towards the front, all the sealer has come up & there's wet flaky rust under that. It's the area the door seal sets against.Then on the outside of the cab, behind the outer rocker an area about 3" x 3" the paint has slipped & rust under that.
So my question is if I get aggressive & remove the seam sealer & the when dealing with the rust I penetrate to the inside how would a person fix this? If it's rusty on the inside how would you treat this? It's about 18" or so that needs attention. The bottom of the outside of the cab corner can be dealt with conventionally.
I'm a diesel mechanic by trade & need all the help I can get with body work!
Oh yeah he wants $5500 for this truck, about $1500 more than I think I would pay for it anyway.
Thanks