I'd stay away from the 03 and 04. 05 to 07 most if not all of the shortcomings were addressed, except for the EGR cooler which in my opinion you should delete anyway. If your state requires emissions just plug the ends and re-install.
But don't let the horror stories scare you from the 6.0s! Most of the ones that had problems are/were owned by neglectful owners.
I absolutely LOVE mine and the only problem I've had was a stuck unisong ring which an italian tune up and custom SCT tunes from Matt @ Gearhead fixed. Nothing but thousands of trouble free miles since then.
Finding one with maintenance records would be a big plus.
I cannot stress this enough. Maintenance is KEY in these engines. And will make all the difference between happy and bankrupt owners.
If you can't find one with all the bulletproofing already done, take the $ required to do it into consideration before buying one that hasn't.
For those "bulletproofed" trucks out there , request reciepts for all the work done. You'll be taking the seller's word for it and you know what they say about that...
Visually inspect it, make sure it has ARP head studs instead of bolts.
They should look like this
You must be registered for see images attach
And not his
You must be registered for see images
Things that should be done / get done in the future.
ARP head studs and OEM head gaskets. Stay away from anything other than OEM gaskets.
Fuel pressure regulator ( Blue spring mod ) crucial if you want your injectors to have a long life.
Dummy plugs and stand pipes. (They can leak and cause no start issues)
STC fitting. (HPOP)
Oil cooler. You want to know the condition of the oil cooler. If the truck has gauges, take it for a drive. Oil and coolant temps should't be further than 15* from each other.
EGR cooler. Get rid of that junkbox. Use it as a target if you're into shooting
Bypass coolant filter is a great +
You'll hear mixed opinions on type of coolant. I ditched the Ford Gold for CAT ELC
Oil & Filter change every 3k
Fuel filters every 5k
Tranny every 30k
Nothing but full synthetics.
Run it like you stole it, they hate short runs and being babied.
Drive it happily ever after....ish