well not really the 2003 as much. it was my dads and we really havent spoken in nearly 3 yrs. but from what he told me he was getting around 15.5-16 with a mix of city and hwy. his had the 3.73 option. i had the 3.42 option and i personally would never buy a truck with those gears again. i was only getting about 14.5mpg city, with tune, intake, and exhaust where is was stock. no hwy was pretty good with about 19-20.
i personally like the way the 06-07classic trucks look with the HD hood on them. id say if you found a good deal on an 03 get that if you like it. just get a custom tune and it will be better.

you could look into an 04 to 07classic crew cab to help out with your son. they all had the 295hp option 5.3 and the option of 3.42, 3.73, or 4.10 gears. im not sure what the most common were for the crew cabs. in the 05 and newer models, probably 3.42's though. you would get a smaller bed than the ext cab.

by the way, i love the 2 door yukon/tahoes. the were cool. i had an 89 k5 blazer, which became the tahoes, years ago.

good luck