I've got 72K on my early 04, which is very similar to the 03 you are looking at. Overall, it has been a wonderful truck and in many ways I'm glad I did not get the 04. 5 truck that seems to have worse mileage and maybe injector problems.
The only thing I've had to do driveline wise to my 2wd truck is replace a left front hub due to wheel bearing failure. This was recent, after 70k. At the same time, I replaced the front brake pads, since it was easy and close to being time for replacement. I've been pulling a heavy trailer, so maybe my brakes have been a little more "tested" than a truck that only sees unloaded duty.
I am now in the process of replacing the HVAC doors. Many of the 03-04 trucks (and some newer ones) are experiencing sticking and breaking of the doors. You may want to see if you can hear a thunk when changing the controls from floor to vent positions. Also see if there is a noticeable temperature difference in right and left side vents. Those may be signs of HVAC failure. My dealership said they have had some evaporator leaks, but I have not experienced it. Make sure your AC is cold.
Good Luck! The 03-04's are the best!!