So my 04 2500 is 4x4, has 6.0 with a 4l80e. 3.73's with a 275/60r17 tire and about 190k on it. Its an LT with a bunch of options.
It averages about 11 to 12mpg. I do tow periodically but its mostly my wife's daily .My cats were plugged so i gutted them. I need to tune to delete codes for cats and rear o2's. When i clear codes with a scan tool it picks up a little fuel mileage. I also converted to e-fans. A/c performance is much better now with idle, traffic, and Florida heat. Something i noticed on mine that i haven't seen on the 1/2 tons is more fuel capacity. Mine has a large side tank and a secondary tank between the spare tire and cargo floor. I have about 38 gallon total capacity. When you top it off with fuel then get out on highway the fuel range keeps increasing for a while. When the gauge does finally start to drop I think its no longer on rear tank and on the side tank. Then you can watch the gauge continously drop.
As far as the roof rack, I'm not sure if would put a kids 4 wheeler up there. It may hold it but i would use a cargo carrier out of the hitch first, less weight to lift over your head. Im not sure on the later models but from 00 to 06 the Z71 equipped 1/2 ton racks seem to be little more stout than the non Z71. The Z71 has roller bar in the rear most bow and and i believe extra bows as well that the standard rack does not have. The roller make things easier to put long items on the roof like a ladder or kayaks. I'm not sure if a 2500 is offered with Z71 package, but something I'm looking at doing is converting over to the Z71 rack.
Another member mentioned Duraburb. Eric is the owner is a super nice guy to talk with. He is located in Apopka, Fl. He specializes in Duramax conversions into Suburbans. Everything is integrated in, all systems work, and looks like it came from the factory with the Duramax. There are a lot of You Tube videos from Duraburb. Been a while since I have looked up the website. Not sure how much info is on there for cost. But again real easy to talk to. He has always done conversations to go with the times. When you Google it, it links to a FB page.