Make sure you get the HO engine which ensures you get the 48re trans instead of the 47re which was availible behind the SO engine. There was also a time in early '03 where a truck could have come with the lighter 47re trans and lighter 10. 5" rear axle. Getting the HO 305/555 engine gives you the heavier trans and 11. 5" axle.
Check for driveline vibes at 55-75 mph. I have had a bad rear driveshaft and a bad front one replaced on different trucks. If the front one on a 4x4 isn't greased every service it can cause the problem even driving around in 2wd as it is always spinning. Sometimes just new u-joints in back will cure the rear vibe.
Ball joints also go bad more frequently than they should, another non-greasable part like the factory u-joints.
Other than that, these are very solid trucks. I have owned five third gen diesels, four with the auto, and loved every one of them. The '03 QC SB was my fav, that truck would get 23 mpg empty and 15 mpg pulling a heavy car on an open trailer at 70 mph.
Great trucks, they drive soooo much better than the second gens, you will love it.
Good Luck