I wouldn't think about paying anywhere near that amount of money for a 12 year old truck.
I still own and use my '03 Duramax and although I might consider buying a new GMC my particular Duramax has been a piece of junk. The engine still runs alright but it overheats. The transmission goes into to limp mode when it is used hard. The radio, power door locks, power windows and interior lights don't work. The battery goes dead if the truck is not used for a few days and I don't disconnect the battery. The truck only has about 150,000 miles on it. Mine probably wasn't the only lemon made. Anytime you buy an old vehicle you need to be prepared to spend some money on it in up keep no matter what the mileage is. The engine in the '05 had injectors that could be replaced much easier than my '03 and the transmission might have been slightly stronger (not sure of that). The "06 was a better truck .... it gained the 6 speed automatic and additional power.