[Overdriven]

The "internet" is half right, rust can creep up on you and reveal more of itself the deeper you dive into a vehicle. The biggest reason for that "internet warning" is "restored" vehicles with rust hidden by bondo and shiny paint. If the cab corners and rockers are as good as you say you have a fairly easy job ahead of you. As was mentioned you can just treat the fenders and doors, prime them and enjoy the truck. If it was me I'd probably just replace the entire fender when you're ready but fix the doors or find good donor doors. Same with the bed.

If you want practice welding, go to an auto body shop and ask them for a hood or whatever out of the scrap pile. Practice cutting it up and welding on it. I'm lucky enough to be able to pick through scraps from a sheetmetal shop (no paint on them) but it's the same thing when practicing. When I started out I only knew the basics from tv shows, learned the rest by practicing on my scraps and amusing myself by making them into sort of sculptures.