My own experience with that era GM pickup, and the Vortec V8s of that era, has been exceptional. Keep them maintained, and they will go for 250,000 miles or more,depending on how well maintained and how hard they are worked.
Only things I would look for are exhaust leaks around the manifolds; there are a few bolts that are known to break on the exhaust manifolds right at the head, and are a PITA to replace.
The 4L65-E automatic is known to be problematic, especially if NOT maintained with fresh fluid, and abused in use. With that said, the 4L65-E is an improvement over the 4L60-E and the 4L60 predecessors.
Other problems to look for in that era GM pickup are: Make sure the gauges all work properly. The gauge stepper motors have been known to be problematic, and the gauge panel itself has been known to have bad solder joints. There is a place in South Carolina (?) that will do a complete rebuild of your gauge package to fix this.
I, myself, would be a little cautious about that rust out problem in the rockers. It may be common in your area of the country, but it would lead me to be cautious of other items that may not have received necessary maintenance (transmission fluid, suspension components, brakes, other fluids). Again, inspect it THOROUGHLY. I would crawl all over the underside of that vehicle looking for rust and other signs of worn suspension, brakes, and steering components.
I hope this helps in your decision making on this vehicle.