Some guys seemed to have had luck with the 6.0Ls south of the border, however north of the border they certainly didn't. Leaking injectors, broken cranks, blown turbos. Even the last round of injector fixes still had issues, and I haven't personally run into a single 6.0L owner who didn't wish they'd stuck with their 7.3 or looked to another diesel.
There were so many issues with the 6.0L that its brought about a rift between Navistar and Ford, a partnership thats been in effect since 1979 (76?). If the 03 that you're looking at hasn't had any of the updates done (turbo, injectors), then I'd stay a long ways away, especially if it doesn't have warranty left.
Rod