Beware!!! once you buy OD green others will start appearing in your driveway soon after. It's like a drug.
Anyway, congrats if you already bought the truck, if not being a M1009 owner myself, I'll give you my 2¢ about them. But bare in mind, it is just my opinion, which is slightly biased being an owner of one.
They are worth what you feel comfortable paying for them. I know that doesn't help much in a way but it is true. You have to keep in mind that this is just like any other 30+year old truck. If it runs and drives, that is a bonus, but keep in mind that it will need work at some point. They typically go from a $1200 starting price on the auction sites to a average selling price pf around $3500, but depending on it's condition and where it is located. One will sell for $6K one week and another just like it will sell for $2500 the next. Private sellers I see asking for around $5000 but also depending on condition.
From the pics, I assume it is a private party sale, since it has tags. These are just like their civi brothers, assembled on the same line, just with slightly different equipment. 24V/12V electrical system, gun rack, push bar, sling hooks, heavy bumpers (heavier than civi stock ones), slave port and radio mount (that's the mount behind the passenger seat)
These can be very reliable if you maintain it, but they are not without their problems as well. I drive mine almost everyday at least 60 miles loaded and it has only had to be towed twice, once for a broken throttle cable and the other for a shorted battery that would not jump start.
The flexplate is an easy fix, but you have to have a good tranny jack to support the trans and T-case. There is a thread to do this without fully taking either out and can still be done safely using 6-7" bolts through the trans but I can't seem to find it at the moment. It's there, somewhere.
The interior is typical for something that sat or has been used quite extensively, it looks original, so I wouldn't expect it to be in great shape. Throw a cushion on the seat and a seat cover for the time being.
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Average mpg is about 16-18 mpg if everything is in good running condition, some report more than 20 mpg, but there are many factors in that.
If the previous owner has any repair history on it, that's great, look for modifications from the stock configuration, this will help in any diagnosis later.
Diesels are not much to be afraid of if you are not used to them. They are just a normal engine with pistons and rods just like a gasoline engine, the only real major difference is the fuel delivery system which acts like the distributor in a gasoline engine. The other difference, no carburetor, you can actually run this engine with no intake manifold, but I wouldn't suggest that.
Don't let the 24V/12V electrical system scare you, it's actually not that complicated, it just seems that way. IMO the 24V/12V system works very well, it can be converted to full 12V, but that in itself creates a new set of problems.
Don't expect much horsepower and torque from these engines as they were not designed for major towing or hauling. Now that I said that, someone will post a pic of their M1009 towing a 13,000 horse trailer, just watch. It was designed as a fuel saver just like it's big brother the 8.2 liter diesel you'll see in medium duty trucks. These trucks will do highways speeds fairly well, but will be gutless if trying to pass in the fast lane.
Rust and body work is pretty much the same as the civi versions, the lower rockers, quarter panels and floors seem to rust easily. Remember, none of these trucks or any vehicle from this era had galvanized sheet metal, so rust is always a factor to look for. The good thing, is that they have almost every replacement panel available in the aftermarket, LMC is a big supplier, there are others.
Being 30+ years old, the springs and the suspension are probably fairly worked, depending on how it was maintained and if anything was replaced over the years, same with the driveline. Steering is decent, better with a stabilizer, you'll hear the term "death wobble" being thrown around. Just check the suspension and steering for typical cracked rubber and worn parts, replace as needed.
The transmission should be a TH400, if not, it has been swapped, most likely with a TH350 which is good but not nearly as good as a TH400.
There is more, but I'm sure others will add their opinions to the thread.
Don't get me wrong, they are great trucks, but don't buy one if you think it's going to be turn key and forget.
Good Luck!! and keep us posted, BTW, this is a great source for help if you should need it.