Just a data point for you on what sound engineering and proper maintenance can do: My dad has an '89 Chevy 2500 light duty 3/4 ton he bought new in 1989. It was put to pasture 3 years ago with 435,000 miles on it, 350 small block/overdrive.
The heads were not off the shortblock until 7 years ago when it passed 380,000. It still has the original, un-rebuilt shortblock in it (it is however dead now). The first trans went out at 180,000 and he got around 100,000 on each trans after the original. Rear differential was rebuilt at 280,000. New radiator every 100,000 miles (glued together, not a great design), new intake manifold every 180-200K (they warp). Fuel pumps around 130k.
It was used in our family run/sole proprietor machine shop. We routinely put 3k lbs in the bed and drove the snot out of it. One month we put 10,000 miles on it - dad was chasing new customers out of state.
Dad was absolutely anal about 3,000 mile oil change intervals and it was serviced at the same trans shop every year almost it's whole life.
My point here is "older" doesn't mean "done", and I wouldn't hesitate to buy a truck with 80K on it and plan on driving it another 100K if you know what you're looking at.
Good luck in your endeavor.