I am thinking of getting a Duramax Chevy or GMC still cannot decide which one... Ext cab short bed 4x4 dark gray leather is what I like

I would probably go with 03-06... I plan on getting it before this time next year

I don't know the difference between lb7 and the lly and the one after that (lbz maybe)? Does one have the egr and the other does not? Obviously the newer ones have more hp and tq but is this good?... I dont plan on doing any 5th wheel pulling just a 6x12 landscape trailer....
Which is better for the mods listed below and more :
guages
turbo back exhaust
tuner- (ppe perhaps)
maybe intake
maybe down the road turbo + injectors

I need help deciding and I guess I have lots of time to decide
Oh ya and I live in Ontario Canada just an fyi
and plan on getting the truck down in the states...