There were a ton of '03 problems - however...
I have an '03 SD Long Bed, 4wd Lariat. I have 145,000 miles on her - 70,000 in the last 2 years. After 6 major oil leaks ( I was starting to understand why Ford put 15 quarts in them - so when 6 leaked out you still had oil in the engine to limp home on), a high pressure oil injection pump (major major work), and 2 injectors - I was actually told by a Ford Mechanic "you seem to have one of the good ones".
Then there was the 30,000 pound Freightliner that ramed me at 60mph in the rear. I walked away. Not a weakness of the truck - rather attains to their strength. I was braking at 20mph and less when rammed. $9000 in damages, my truck is back together and all things well.
If you can find a '05 - you would have less chance of problems.
As far as my '03 - once all the "issues" were addressed - leaks fixed, new oil injection pump and new injectors - I really do love my truck and short of a crew cab - would not think of getting rid of it for anything else.
See if you can get an Oasis report from Ford for it. Also ask for a repair log.
Any dealer should be able to run one of them for you even if it is a private seller.