I had a 1997 6.5TD in a K2500. I really liked the truck. The 6.5 ran strong and was fairly easy to work on. Besides the PMD, I'd say there are two other components you're going to want to keep an eye on. The first is the harmonic balancer. Over time the rubber isolator will degrade to the point that the balancer will rotate about its hub putting the rotating assembly out of balance. Stock replacements are relatively inexpensive and good insurance against a broken crankshaft. I don't remember who makes it, but an aftermarket co. makes a performance replacement that is still cheaper than a replacement motor and will last just short of forever.
The other key piece to keep an eye on is the oil cooler lines. Over time with those, the crimp connections will loosen up to the point that the rubber line will pop off of the fitting. From what I've read, that will mostly likely happen at speed and by the time you realize the hose has blown and you lost oil pressure it will be too late and the engine will be toasted. Other 6.5 owners have retrofitted aftermarket lines and fittings that won't blow.
As BBJordan said they do have a timing chain which will wear and need replaced. I replaced mine at 230k miles and it brought decent power back into the truck.
Having said all of that I would own another one. Once you know what to expect, they can be reliable and relatively inexpensive engines. The forum I primarily used when I owned mine was
www.dieselplace.com. There is gobs of information on there (much like oilburners for the idi Fords) to help you keep your truck dialed in and reliable. Ignore the ******* at work. More than likely he's talking out of his ass anyway.