Take a look at how well it was cared for. If there isn't a lot of rust, if electrical looks sound, if it drives well (suspension & steering), brakes well, and if it wasn't hot rodded or towed most of those miles ...you probably don't have to put a lot of money into it.

If there is excessive rust, then you'll have lots of issues (hubs, rotors, ... lots of problems). Otherwise the general maintenance may not be a big deal for a great truck that may last you another 15 years. (Assuming it has an Allison transmission, you'll be good for pretty much the whole powertrain).

I'd inspect it and take note of fixes that are needed. Then I'd take 50% of the cost and cut that off the final price. This should be fair.

Some things to look for are ball joints (needs to be lifted to test wheel wobble), and tie rod ends (visual inspection of boots for tears or grease all over). Look under the hood at hose condition (cracks and leaks). Look underneath for dried coolant that had leaked (may be water pump issue). Inspect for excessive oil buildup around engine (gasket), and transmission pan and differential cases for active/new leaks. Inspect all shocks (including steering damper) to ensure it doesn't look like they have leaked anything. Try to wiggle the drive shaft; if there is play the u-joints probably need replacing. Without the engine or power steering engaged, turn the steering wheel back and forth about 1/8 turn; there should be zero play in the steering wheel where it doesn't turn the wheels. Inspect the wear on the tires; are they wearing evenly? If an edge is more worn, this could be due to a number of issues including lifting the vehicle and not adjusting control arms and sway bar. If it has air suspension in the back, inspect the bags. Check leaf springs to ensure they are aligned. When you turn on the truck, listen if the engine sounds excessively clackity-clackity (valve adjustment). Check that the AC and heater work. Is the fan excessively loud at the middle settings? (May have been a failure and an aftermarket replacement).

You also might be able to subtract $ for the condition of the tires. Cracks in windshield?

Anyway, these are some simple things you can quickly check without having to take it to a shop.