i would walk.. i found my 95 ext cab dually with 112k on it and got it for $4,300 like 4 or 5 years ago. what your going to look at is a work truck that nobody really cared about taking care of. as long as it drove, it was out working.
things to beware of - plows will put alot of extra stress and wear on front end parts, esp if its not a factory option which most likely it wasnt. the transmission has definately seen alot of work, which is a $4,000 item to replace in itself if its an automatic, not sure about manuals. if its a dually there is prob no 4wd, so at least there is no front diff , locking hubs, or transfer case to worry about.
look under the truck for rust. the number 1 place to look is at the rear hangers for the leaf springs and shackles. if they are rusted, or have holes in them they will also need to be relaced. factory ones are rivited in, if they were ever replaced they will have bolts. this in itself is not something that would turn someone away, but its one more thing to add on the list of problems.. theres no sense in getting a good deal on a used truck if you gonna have to spend more than you paid to have it all fixed.
honestly after having a truck like this for a while now i would probably not consder a rusty plow truck as a buy. the plow and the dump kit are probably worth about half of what they are asking for the truck alone, and im sure there is good reason for that. these trucks are getting cheaper to buy, buy are still expensive to fix sometimes. so you can pay now, and pay later too - or just find something better. one option not to overlook is finding one a little further away from where you are. the right truck is worth driving a few hours to look at, and having shipped home if need be. i think i paid $350 to have s truck shipped from Mass to NJ.
do you NEED the plow? do you NEED the dump? ( although that might be fun). is this going into a fleet to work or just going to be a daily driver?
search youtube and there is a video or two on how to buy a used powerstroke. good luck