With Banks you will never get the full potential out of your Duramax, they are good units for people who tow alot but if you want fast a flexible EFI is the only way to go.
As for your tranny I would look into a good stage IV kit from Mike L. he can get you pointed in the right direction.
I would check the service records of the truck and see how many HG's it's been through before I drove that far. I have eard the 6.0 got better as the years went with the HG issues.
If you just want the motor's I will take the K5. Generally these trucks are in pretty good shape. The AirForce ones tend to be the nicest. We have bought duece and a halfs up there in the past. The k5
s and pickups will either go for penny's or for a ton of money. I have seen them sell...
I love the 12 v trucks. They just run good and last forever. It's a pretty truck. Even the "Dreaded B blocks" are not that bad. It's a casting issue but no big deal unless you are gonna be makin some big #'s. Also I beleive it only effected 97 trucks, I could be wrong though.
Remember how bad the market is right now, I am not sayin you won't get 20K bu it will be tough. If you have time you can sell it for what you are askin.