Google will send you to the duramax forum. A lot of good info there.
Look at the exhaust manifolds and look for where someone may have installed EGT probes. If they did I would stay away. Was more than likely tuned and abused by a Bro.
I am seeing diesels start to come down in price in my area because of the repair costs that come early on in the life of the truck. Just not worth the expense and headache if you don't need one. If pulling 10K plus a week you need it. If only for the occasional tug to the campground you don't.
Look up prices for injectors, pumps, and tranny rebuilds. If you can't do these repairs yourself, add in labor. Gets expensive quick.