motox,
I am going to go against the grain here and say personally, I would go with a 6.0 with no hesitation. I say this not because of the "problems I have heard about, or the "they are all junk" mindset. I know several of the responses are from guys that know.
I agree, the 6.0 had growing pains, and I would say most of them have been taken care of my now. The newest 6.0 is 4 years old. I also would not say to avoid an 03, I have one in the garage right now. I have owned it for almost 4 years and it has been DEAD NUTS RELIABLE!!!!!! The only time it has not started is due to low batteries due to a bad alternator. I have 110,000 on it. I have done regular maintenance, and replaced the bad alternators (long story, but not the engine or trucks fault.....
I would say what to look for is not one that has all the above listed parts removed. I would look for one that is still stock... NO PROGRAMMERS, CHIPS, etc!!!!! All those parts do not have to be removed for it to be reliable. What is the most important on the 6.0 is the maintenance. The oil should be changed at 5k..... dino or synthetic, the injectors shear the oil in normal operation, and it murders the oil. Fuel filters at 10k. Use ONLY MOTORCRAFT filters!!!!!!! Get an OASIS report if possible, if it has not been a problem child, if it was, has been repaired and has been fine for a long time after, I think it should be fine. In stock form, they are powerful, and reliable. This is said from someone with ownership experience. Also, the 6.0 does not like to be babied, work the truck, at least once a week, drive it a little hard, not abusive, but a few good hard accels will keep the EGR Clear and the turbo clean. BTW, my truck is stock, has never had a tune other than the factory tune, and no other mods. At 110,000 miles, I have the factory headgaskets, EGR Valve (has not been removed from the engine ever), factory EGR and Oil Coolers and the factory turbo (has not been removed from the engine ever). Do some research, and see what info you can get on the truck you are looking at, 7.3 or 6.0, and you should be fine.