Can't tell you how many trucks we looked at that looked nice in picture but when we got their oh boy rust. We eventually just started asking for pics where we knew the rusty spots would be. You know under the back of the sleeper sill, cab mounts, sleeper & cab rails
Pretty common sight here in the northeast. I think that's the going rate for having it sandblasted and painted, a lot of work if it's done right. I would pull the rear wheels off and knock all the rust down with a needle scaler and go over it with POR15. Funny how frame rust is a never ending battle on some trucks yet others can look like new 20 years later and be from the same area.
I think some people don't wash their truck enough. It's like a boat, when you go thru salt water, gotta run the motor thru fresh water, so the salt won't rust it away
Easy the ones that look new were taken care of. The 99 we got down in Texas never left Texas pretty much and looks better than most 5 year old trucks up here in the rust belt
Another thing the OP might want to take into consideration is the wiring on that truck since it looks like its seen its share of elements. I would plan on replacing a lot of it if it hasn't already been. Probably a lot of corrosion in there. I did most of my forward harness over this past winter and was amazed some of it was even working. Some pieces broke apart and crumbled into green dust.
It's hard to tell exactly from the photos but I would be careful. If you sandblast it or have someone sand blast it you may find that the frame is worn so thin the rails may have to be replaced. The same with the differentials. They can rust so thin to where you'll go down the road and they crack. I'm not saying to not buy the truck, but really look it over carefully and especially at the spring hangers in the front and between the suspension in the back to make sure there are no cracks in-between that rust. I would try to buy it for an amount of money that if you have to replace the frame rails it's worth it, or that you can put that engine in something else and maybe use that one for parts if worse comes to worse. Frame rails and differentials can be expensive, on top of anything else the truck might need.
OEM rails undrilled were like $9k each for my KW. I had ones bent up by a steel shop for like $50/foot.
its really hard to tell from the pics just how bad it is but my impression from the pictures is it would be a firm pass for me if it was me looking to buy the truck.