The 2003 6.0 diesel's along with 2004 were problematic with Ford. A lot of people had injector problems and head gaskets and other various issues. Check the history of the truck at a dealer if the injectors and head gaskets have been done. If yes, it would be good, if not, its a matter of time before you get your wallet out.
My 2003 had computer problems, it had to be reprogrammed almost every two months by the dealer, but I did have one of the first ones out. When I sold it, I knew that the head gaskets were shot and I had two faulty injectors, but it was still under warranty so the new buyer had it done.
My 2006 is an absolute dream, wouldn't want anything else.
I did have a F350 so it is a little lighter than the F450 and it is geared differently. I had high speed rear end (3.73) and I was averageing 14mpg city and 19mpg highway. I was chipped and was putting down over 400 HP on the pavement so the linkage at the kneecap would freeze up sometimes and kept the accelerator pedal to the floor, so it could be a little lower than that at times
Dan