Check out
www.cumminsforum.com
http://www.dieseltruckresource.com
Both have lots of good info
In 04.5 they changed to the 325 hp and 610 ft lbs
A few more injector problems but in 05 or 06 they changed the valve seats
I've heard a few of the valve seat problems on the 03-04 on the internet, but never talked to anyone that has had it happen. A lot of people are saying it is caused by high egts. So those with programmers who don't watch gauges or those who tow 30k are prone to problems. If it did happen to me, I would say upgrade to the 06 head, stud and o-ring it. Hopefully you could get away with a new piston and call it good.
Any of the 03-07 trucks are pretty problem free. Biggest problem is probably the lift pump, but that is minor and easily fixed.
Hear about U-joints, ball joints and injectors here and there, but I don't consider them problematic.
Also, don't forget that some of the trucks that have had engine problems are usually heavily modified. There are a lot of trucks putting out 600-700 hp or more.
Theres lots common rail trucks on those forums that have over 300k and never have had the valve cover off.
All being said, I love my 03 and would not hesitate to buy any 03-07 Cummins.
As Doc mentioned the 7.3 isn't bad at all, just not much power. Run far away from the 6.0.
The 01-04 LB7 Duramaxes are good although they did have the injector recall. I would stay away from the 04.5-05 LLY IMO. The 06-07 are in between.
I personally would prefer an 99-03 7.3, 03-07 Cummins, or 01-04 Duramax over any of the new trucks with DPF and EGR. But thats just me.