They are known for a few problems with the engine, no power house either.
thedieselplace.com is the place to go for 6.5L info, i had a 6.5N/A that was trouble free but have heard horror stories from some.
The engine is heavier than the gas alternatives in the 1500 series, something to think about if buying for a plow rig.
You will most likely need timbrens on the front if your planning on putting a plow on it. To give you an idea. Last year I paid 7 grand for a 1995 K2500 with a 6.5 diesel in it. The truck is bare bones, no options, but came with a 525 Snowex spreader, and an 8 foot Fisher minute mount 1 plow. Oh by the way I know I got a deal, but the truck only had 57,000 orginal miles on it as well. So to answer your question, I think 6800 is high on that truck. I would guess more along the lines on 4500 or so.