As stated above, the 6.0s can be a hit or miss. I feel very fortunate that I got a good one. I haven't had to deal with any of "the big" problems yet. (knock on wood) I have an 04 with almost 117,000 miles on it. I have dealt with an EGR valve issue, but fixed that and my custom tunes turns it off now anyways, and FICM (fuel injection control module) that went bad. Both things were things that I was able to repair myself, as just about everything on our rigs, and saved A LOT of money doing so. I put my truck USE! I pull heavy loads with it at work, wheel with it in the mountains, and drag race it. I love it!
As far as fixing the problem right away.... Up to you. If you have the money and want the instant piece of mind, then go for. I would suggest using stuff from Bullet Proof Diesel. The have pretty much addressed all the major issues with the 6.0, ie... EGR cooler, oil cooler, and are developing a new FICM as well.
What years are better than others.... That can be debated all day long. 03s came with the round EGR cooler that is said to be better and more reliable than the square type. The 03 came with a 10 vane turbo and the 04-07 came with a 12 vane turbo, the only difference is the whistle that the 10 vanes put out, pretty cool in my opinion. Later year trucks had different strategies that were supposed to make for better cold starts and other engine parameters. I do not know the details on all of them. I do know that the 04 strategies have been said to have some of the best as for as power, performance, and throttle response. That is one reason that I will not get my truck re-flashed.
Make sure you do regular maintenance with all the right stuff. Oil type can be debated, just make sure it's good stuff. Make sure to use Ford gold coolant, not the green stuff that 7.3s can use, or do a complete coolant flush and switch to and ELC coolant.
Hope my words help in some small way. I am by no means an expert on these truck, but have learned quite a bit in the several years that I have owned mine. Plus, when you hang around a bunch of Power Stroke addicts, like John (Rogue-Texan) and I do, you tend learn a thing or two pretty qucik.
