I bought an 05 two years ago with 50k on it, it now has 104k on it 90% of which we're towing. I love the truck and plan on keeping it till I hit 200k. Small things will start to go wrong with any truck but I wouldn't be worried about the drivetrain.

If you can pick it up for the right price grab it. Just realize you have a $ 3k bill coming at 350-400k miles.

Just my opinion, however I own a 05 LLY Dmax myself. I bought it from the original owner with 85k on it. It was used for towing heavy loads and plowing before I bought it. I had several small issues with the truck, however nothing that would steer me away from buying another one. It now has 140k on it and I tow and plow a lot with it. The drivetrain is as tight as the day it came off the showroom floor. Any truck will have issues with wearable parts. If the proper maintainance was done I wouldnt worry about buying one with 200k. My buddy has over 350k on his 03. He is on his third set of injectors, other than that the drivetrain has been bullet proof for him. This is my first diesel, and I will def buy another Duramax, hopefully i wont need a new one for a long time

+1 My best friend has an '05 2500 Dmax, bought new and has 180k on the clock. It's had regular maintenance and has had a couple of steering parts, new rotors and an axle seal replaced. Other than that, tires and fuel is all she'd had. The only thing showing any wear is the drivers seat. I swap trucks with him and tow from Dallas to Gulf Shores Al at 1900 RPM getting 13 MPG @ 75 MPH. Awesome Pickup.

We have a 06 at work, 345k tires/brakes/cpl wheel bearings, just normal stuff, but at 345k Ny winters are getting to it, as anything else, Injectors/glow plugs are the biggies, ours has a few glow plugs out, will not come out due to age/ny winter, but she keeps starting and running,

I run a small car lot that specializes in high mile diesesl trucks among other things. A few weeks ago I had enough duramaxes on my lot to point to one with around 100k, 200k, 300k, 400k, and 500k all next to each other. I just sold my personal 04.5 with only 370k on it and upgraded to an 05 with only 240 on it. The truck with 500k had a torn drivers seat, but still drove great and pulled my 15k car trailer with no problems. Today I just sold an 03 with 486k on it to a guy just so he could tow his boat with it.
The only truck I had to do major repairs to was my truck with under 200k; once you get past that point somebody else has already done the work for you saving you the money needed on repairs.
I would say stock transmissions are good for most of the life of the truck IF you don't throw a tuner on it or abuse it. You can get a performance built transmission for around $3500 which will need replaced and allow you to throw a big tune on it. Then you will have more power, easier towing, and better MPG as long as you aren't keeping your foot deep in the throttle.

Also, if you want the best price on a diesel truck, let me know. I've got an 07 duramax with 198k on it for $14,999, an 05 fully loaded 220k for $14,999 and an 04 with 330k for $8000.