DRIVE IT DRIVE IT DRIVE IT!
Seriously, drive it! Don't tear her apart before you've enjoyed it some and work on stuff as you go. I'd replace the cab as well. They make all sorts of patch panels, but when the roof/driprail gets that bad... the cab is probably shot
steveshow had some great tips. You can get a second cab and basically get it ready for paint before you even have to touch your drivable truck. You can fix the tailgate/bed/interior on weekends and still drive it during the week.
Most importantly though, go through the suspension, engine/drivetrain. Replace any worn stuff and enjoy
Things that typically wear out and should be replaced are the trailing arm bushings, cab mounts, and for the front end the control arm bushings, ball joints, and tie rod ends.
You already have power disk brakes, power steeing, auto, and A/C, heck you're good to go!
Enjoy!
I'd also like to add, I'm originally from the rust belt and I've seen some bad cabs in my time, but I don't recall EVER seeing one with the inside back wall of the roof rotted out!