check the history of the 03. the early 6.0s had a LOT of issues with the glow plug wiring and AC lines. seems they chaffed on the valve covers. if they've all been done and covered, you're probably good.
another big issue was the early transmissions didn't have a hardened snap ring for the reverse gear. when it broke, reverse engaged and it would shoot it out and crack the case. if it hasn't happened yet, the PO was either REALLY lucky, or....