I think if I were looking at a used plow truck, the first thing I would check would be the transmission fluid. If you pull the dipstick out and it's burnt, that's a tell-tale sign of trans problems to come. Also, look the front section of the frame over REALLY well. Look for irregular bends and cracks. If you can, also check the fluids in the transfer case, and front & rear differentials. If the fluids aren't up to par, you should probably move on. I bought my truck without a plow, and had it put on later. I've changed my trans fluid about 6-times in the last 13K miles. Overkill... probably. But if it does fail, at least I know it wasn't because of lack of maintenance.
I would take the truck to a dealership that performs inspections. Just by them putting it up in the air, they can tell alot about how well the truck has been maintained, and give you a better idea of how long it will last. If it costs you a few bucks, so be it. You'll be better off in the long run.
-Tim