I have a 85' M928 (with winch), it was repainted in 2007, is a VERY nice looking/driving truck. Has the spin-on oil filter conversion, fresh oil, coolant, and trans fluid. The bed is not a dropside (I have not seen one with a dropside so I agree with VPed those are probably hard to come by).
I would take $12,000 for it and pretty firm at that. I dare say without going to an govt auction site you won't find a nicer one cheaper.
http://www.steelsoldiers.com/showthread.php?117262-1985-M928a1-Long-wheelbase-long-bed-5-ton-6x6
At auction, I paid ~$1k more for mine than another guy did for his (same day, same location), mine looked nicer by far, his was pretty rough looking and has a bad engine knock, I don't think he has figured out the issue yet. BOTH trucks were auctioned as non-operational, so it is a crapshoot if you buy one from a govt auction. For a nice looking truck (927 or 92
in RUNNING condition, I have not seen one auction for less than 6-7k recently.