I am personally a huge fan of the 5.9 cummins. My buddy owns a 6.0 powerstroke, and has rebuilt it 5 times now. That being said, the 6.7 Powerstroke is not a bad engine....I still, however, feel the cummins is the best option...
1) 5.9 Cummins, 12 or 24 valve, doesn't matter. They are by far the best diesel engine, there are plenty out there that have got a million miles or more on them. Unless you are looking for 700+ HP, they are very reliable engines, and even stock they have power. Plus, they sound awesome!
2) It really depends on your budget honestly. A 2nd gen Cummins with 100k is hard to find, and if you do find it, it will be expensive, even if it's all stock. Personally, I wouldn't be afraid to buy a Cummins with 300-400k on the clock, (unless its a 6.7, which in my opinion don't last quite as long), its the dodge truck that I would be worried about. Dodge quality is infamously crappy, and any stock parts over 150k miles would need to be replaced. You also have to take into account drivers - if it is a completely stock truck with 185k miles, and the owner was an old man who used it to get groceries, it would be in far better condition than if a truck that some kid has put a tuner on it, put big tires on it and done burnouts everywhere. IN general, the more modified the truck, the more wary you need to be when buying.
3) I own a 2003 Dodge 2500 (3rd gen). Its the 24 valve, and is the first year they went form injection pump to fuel rail. I bought it about a year ago with 189k on the clock, from a guy about 8 hours north of me, and I LOVE it. I specifically wanted a truck that had all the crappy dodge parts redone - steering, suspension were the two biggest. I also wanted a truck with a relatively stock engine. I found one. It has a smarty tuner, but its a manual, so tranny issues dont really happen. I drive it like a granny for the most part, except when a ford owner thinks hes going to beat me lol. Since I have had it, I've replaced the power steering pump, and the back brakes. Thats it. BUT, with 195k on it now, and stock injectors and turbo, it is starting to have white smoke instead of black (injectors are dying), the turbo blade edges are scuffed (shaft play) and I'm a little worried about the head gasket. To replace, and upgrade the injectors and turbo is going to be around 6k. I don't have 6k. So thats an issue.
Final word; Cummins is the way to go, but they can be EXPENSIVE. Choose your truck wisely.