As mentioned above, if an LBZ, get it now! That year had the 6 speed Allison, that eng/tranny is a winning combination.
If it is an LLY, it is still good. I had an 05, worked like a charm. I never had injector problems nor the head gasket problems, though, I did by it used. I put nearly 40k miles on mine in the 2 years I owned her, with only oil changes and fuel...
Check the bottom for rust. From 1999-2006, the brake lines, fuel lines (and cooler), and transmission lines would all be rusted if it was a northern truck. I found this out the hard way, it was going to cost $10k for parts and labor to change everything that was rusted beneath, I decided to trade it at that point.
Cale