Marlin, welcome to the Stovebolt.
I think you've got the basic information on what to look for in buying one of these old Big Bolts.
That said, I'll just bet that whatever hasn't been talked about here will be the first thing you will have a problem with.
If the owner will go with you and let you drive the truck, I would suggest you do things like stop while going down a hill (not a steep one, just downgrade), then put the old girl in reverse and back up (you can even test the emergency brake--will it hold, let a little off the clutch and slowly release the emergency brake). A smooth start is nice; if it bucks and kicks backing up, think, maybe, clutch problem.
Check the windshield wipers, all the lights, dash lights, go back and forth between high beam and low beam a couple times, anything you can think of. None of these items are deal killers, but it helps to know what you may need to work on first.
While you are driving the truck around on a straight road, do a series of wedeln turns in the lane (if you are a skier, you'll understand). Great for checking the feel of the control slop in the steering, caster, and camber. Don't forget, this is an old truck and there will be a different feel from a new vehicle, and wear in the steering as well as tires will let the truck wander and hunt a little, but, if you have to "chase" the truck with the steering wheel to keep it in your lane, well, maybe it ain't bad, but it ain't great, neither.
Look up underneath the fenders, running boards, and such. Dirt is ok, and Oregon trucks probably have lots of dirt and mud, but rusty holes and bubbling metal mean future work.
Look at the oil and the grease (in the spring shackles). Now, some hawnyock can take a grease gun and go out and pump the fitting full for the first time in ten years, but anyone who doesn't do basic maintenance on a truck just ain't gonna figger to take the time to do that.
Finally, fall in love with the truck, don't let nobody else buy it, and send us more pictures.
Get with your accountant. Capitalize the truck as a business asset and get your customers to pay for your fun of deliverin' their furniture.
