You don't say what year or mileage Sub that you are looking at. This would help to narrow down the advice. I have a 95 3/4 ton 4x4 sub with 330Klms on it. The PO had the motor replaced about 40K ago The replacement motors with the updated blocks are much more reliable. The PMD can be remote mounted to virtually eliminate the IP failures. The 97s have improved water pumps and dual tstats which help with cooling issues, this can be retrofitted to any 6.5.The turbo wastegate control can be replaced with a simple adjustable spring device. There are a few fixes for the various problem areas which mostly solve the issues. Check the GM diesel forums at
http://www.gm-diesel.com/diesel/vbull/forumdisplay.php?f=6
for discussions of the various issues. I like mine and there are almost no other similar vehicles around which have the seating or towing capacity. Cheers Dan