The 6-speeds from what I've heard are pretty rock solid. The Allison is a great tranny providing you don't cross the 750FTLB mark and drive it like you stole it. I'd say the 02 should still get within the $18K+ range, they still have a good re-sale no matter the market. Depending on cab, size of bed, and options, they'll still be in the upper teens. Here's a KBB example price:
02 Crew Cab Short bed 4X4 in Good condition with the standard options
$16,000 at the moment.
You'll be happy with the Duramax trucks, they respond very well to an exhaust, intake and programmer.