I've had 14 work trucks with the International T444e engine which is very close to the 7.3 Powerstroke (I think the ECMs and oil coolers are a little different) and they've all been excellent engines. They've all gone at least 300K in hard use trucks run at 18-20,000 lbs for 10-12 hours per day. None of them had any mechanical issues until beyond the 250K mark and then it was usually the injector pump.
My personal truck is a 2005 Duramax, so a newer version than what you're looking at, but I did have one 2003 work truck with the Duramax in it a while back. My personal truck has been excellent and I haven't had any mechanical issues buy it's only at 140K. The work truck was a light duty (10,000 lbs) rig that ran right at 10K lbs every day. It got better fuel economy than the 6.0 L gas engines but it spent an awful lot of time in the shop. Injectors, head gaskets and glow plugs were a constant issue until I ditched that truck at around 200K.