Does TN have yearly inspections?
If so, there is a record of the mileage from each one of them.
(I was not trying to show you vans to buy BTW, I just wanted an idea of how much modification you were talking about. The link and pics you provided snapped it all right into focus.
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That is a pretty well-done setup. It is hard to put a value on it.
The trans is a C6, so no planetary 'shudder' into OD each time. That really is about the best setup there was of the era for a diesel van.
Whoever made that rear box...they did you a favor. It confirms it has not been owned by an abusive kind of guy.
The FIRST time that thing went even a little off road or jumped a curb, that box would be mangled.
The paint also looks too clean in the front to be 142,000. I would have to see it in person to tell if it is a repaint...but if it is original that very well could be original miles. 142,000 would have pitting on the nose and windshield area.
The dash and the door closing are also hints. Those pins are not worn out. Unusual in that year van.
Actually, I am pretty damned impressed with it from what I can see in the video.
Can you bring it to a diesel shop? As in...a place that works on Navistar?
I would spend a couple hundred and have them go over it top to bottom if you are considering it.
Know one thing about that kind of A/C unit installed in the back. I think it is the kind that uses inside air to blow out the vent hose.
They do not work well for vehicles, because they suck air in through the windows to blow out through the coils.
In a house, this is okay...but in a cab that small, you end up sucking in more heat than it actually cools.
Make sure you run it to see what I mean. If it's the "one hose vent" model, it will be apparent instantly.
Figure on checking/replacing hoses/brake flex lines/belts. Get a date on the tires. Over 5 years old, toss them.
(Those are pricey.) When they sit, the calipers often will end up sticking after a small amount of use.
If these items are dry rotted, the value goes down.
If a grease powered van is your goal, it could be worth close to what they want.
One thing for sure...that is a very rare bird to find in that condition. It would cost thousands to set that up after buying a bare van.