the Lb7 can be very reliable, the biggest issue is the injectors, I would get the VIN checked to see if they have been replaced under warranty, if they were replaced after 04 i believe they will have the upgraded bosch bodies to help prevent cracking
I can never repeat this enough, Research Research Research!!!! Learn absolutely as much as you can about model of truck/engine/trans before you buy! that way when something does come up after you bought it, it wont come as a huge suprise
My advise would be to go down to the LB7 section http://www.dieselbombers.com/chevy-g...max-01-04-lb7/
and start reading posts, it will give you an understanding of what other owners have had issues with, how they fixed them... etc.