Fixing that rocker situation isn't real bad if you have some basic knowledge & tools needed to do the job.
What I have learned over the years of doing/re-doing these trucks is: Most people are "afraid" to cut the new replacement panel to the "correct" size & just stick it up there & start welding...which looks to be the case here. The notch in the back was only done since it's a crewcab & the shape was wrong.
The replacement rockers work best/easiest if you can put the seams in the rounded corner outward (either square or at an angle) using only as much of the jamb area as needed, but using the entire outer. I would start by grinding through the butched panel right on & below the weld line with something like a 4" stone wheel, remove the old one & plan from there. I've done MANY that had previous rockers, cab corners installed over top & just removed them, cut the new rocker in the "correct" location & smoothed up their weld that was 6"- 8" higher & moved on.
I would look at the floor, cab mount, ect. hopefully our guy didn't need to do much there!
As far as the rust around the striker...I've seen some. I think I've seen rust just about EVERYWHERE at one time or another...looks like from what is in the picture, it's likely solid & just needs sandblasted & smoothed up. Lorne