Try going by with 5 crisp Benjamin's to make an inquiry as to what he wants to do with the truck. The sight of money makes the offer more real and immediate.
I could go on about what I paid for my first M37 ($300) and it ran, and it had troop seats, and a soft top cab frame, and a bent drive shaft. But that would not mean much as all that happened back in 1963. Since then, I have purchased several other M37's and so has my son. Mostly we buy parts trucks to have lots of spares for our trucks, but we have also fixed a couple up for resale. Looking at the value of the truck for parts, I see an engine core worth more than a civilian Dodge 230 (there are differences in the block between Military and Civilian.) I see at least one really straight running board. Very often, these are rolled up from being off road in rocky terrain. The soft top cab frame looks like it is all there. Just from what I see in the pictures, there are more than enough parts to cover $500. I think your offer is safe and makes sense. ~~ A year and a half ago, I would have said that an offer of $1500 would not be out of line given the proviso that the engine would run, the radiator was at least repairable and there were no broken or stripped gears to have to replace. I would think that another $1500 for proper seats, a cab canvas, some windshield glass, distributor, regulator and generator plus a lot of elbow grease would have given you a decent usable truck worth $3500 to $4500. ~~ Today, who knows what something may be worth tomorrow or the next day. You have been pitched more than once with a running truck price of $2500 (all be it not in your part of the world) that is about 2/3rds of what something like that would have brought a couple of years ago (assuming it is as represented.) I think your approach is properly cautious and if successful, it should get you underway with the addiction the rest of us have so badly.